Table of Contents

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

Form 6-K

Report of Foreign Private Issuer

Pursuant to Rule 13a-16 or 15d-16

of the Securities Exchange Act of 1934

August 2, 2013

Commission File Number: 1-15174

Siemens Aktiengesellschaft

(Translation of registrant’s name into English)

Wittelsbacherplatz 2

80333 Munich

Federal Republic of Germany

(Address of principal executive offices)

Indicate by check mark whether the registrant files or will file annual reports under cover Form 20-F or Form 40-F.

Form 20-F x Form 40-F ¨

Indicate by check mark if the registrant is submitting the Form 6-K in paper as permitted by Regulation S-T Rule 101(b)(1):

Yes ¨ No x

Indicate by check mark if the registrant is submitting the Form 6-K in paper as permitted by Regulation S-T Rule 101(b)(7):

Yes ¨ No x

Indicate by check mark whether by furnishing the information contained in this Form, the registrant is also thereby furnishing the information to the Commission pursuant to Rule 12g3-2(b) under the Securities Exchange Act of 1934.

Yes ¨ No x

If “Yes” is marked, indicate below the file number assigned to the registrant in connection with Rule 12g3-2(b): 82-

Table of Contents

TABLE OF CONTENTS

INTRODUCTION

Siemens AG’s Interim Report for the Siemens Group complies with the applicable legal requirements of the German Securities Trading Act (Wertpapierhandelsgesetz—WpHG) regarding quarterly financial reports, and comprises Condensed Interim Consolidated Financial Statements and an Interim group management report in accordance with section 37x (3) WpHG. The Condensed Interim Consolidated Financial Statements have been prepared in accordance with International Financial Reporting Standards (IFRS) and its interpretations issued by the International Accounting Standards Board (IASB), as adopted by the European Union (EU). The Condensed Interim Consolidated Financial Statements also comply with IFRS as issued by the IASB. This Interim Report should be read in conjunction with our Annual Report for fiscal 2012, which includes a detailed analysis of our operations and activities.

Due to rounding, numbers presented throughout this and other documents may not add up precisely to the totals provided and percentages may not precisely reflect the absolute figures.

1

Table of Contents

KEY FIGURES Q3 AND FIRST NINE MONTHS OF FISCAL 2013^1,2

unaudited; in millions of €, except where otherwise stated

1

Orders; Adjusted or organic growth rates of revenue and orders; Total Sectors profit; ROCE (adjusted); Free cash flow and cash conversion rate; Adjusted EBITDA; Net debt and adjusted industrial net debt are or may be non-GAAP financial measures. Definitions of these supplemental financial measures, a discussion of the most directly comparable IFRS financial measures, information regarding the usefulness of Siemens’ supplemental financial measures, the limitations associated with these measures and reconciliations to the most comparable IFRS financial measures are available on our Investor Relations website under WWW.SIEMENS.COM/NONGAAP

2

Table of Contents

INTERIM GROUP MANAGEMENT REPORT

OVERVIEW OF FINANCIAL RESULTS FOR THE THIRD QUARTER OF FISCAL 2013

(THREE MONTHS ENDED JUNE 30, 2013)

Management’s perspective on third-quarter results. Our previous expectations for our markets are not likely to materialize. We therefore no longer expect to achieve a Total Sectors profit margin of at least 12% for fiscal 2014. We will continue to rigorously execute agreed measures under the Siemens 2014 program, to close the gap to our competitors. With the listing of OSRAM and sale of our stake in NSN, Siemens already has a clearer strategic focus.

Significant rise in orders, slight decline in revenue. Third-quarter orders rose 19% year-over-year, lifted by a higher volume of large orders. On a comparable basis, excluding currency translation and portfolio effects, orders were 21% higher. Reported revenue was 2% below the level a year earlier, and on a comparable basis revenue was less than 1% below the prior-year level. The book-to-bill ratio for Siemens was 1.10, and the order backlog (defined as the sum of the order backlogs of the Sectors) increased to a new high at €102 billion.

Orders climb on major contract win. Orders in the Infrastructure & Cities Sector rose on a contract win worth €3.0 billion for trains and maintenance in the U.K., one of Siemens’ largest train orders. Orders for the other three Sectors were close to their respective levels in the third quarter a year ago. On a geographic basis, the region comprising Europe, Commonwealth of Independent States (C.I.S.), Africa and Middle East reported sharp order growth due primarily to the large order mentioned above. Orders came in lower in the Asia, Australia and Americas regions. Emerging markets (according to the International Monetary Fund’s definition of Emerging Market and Developing Economies) on a global basis grew 6% year-over-year, and accounted for €7.096 billion, or 34%, of total orders for the quarter.

3

Table of Contents

Revenue development shows mixed picture. The Sectors Infrastructure & Cities and Healthcare reported higher revenue compared to the prior-year quarter, with a majority of businesses in each Sector contributing to growth. These increases were more than offset by revenue declines in Energy and Industry compared to the third quarter a year earlier. On a regional basis, revenue rose in Asia, Australia and Europe, C.I.S., Africa, Middle East. In contrast, revenue fell in the Americas due primarily to the wind power market in the U.S. Emerging markets on a global basis reported a 2% increase in revenue year-over-year, and accounted for €6.458 billion, or 34%, of total revenue for the third quarter.

Update on “Siemens 2014.” In the third quarter, Siemens continued to implement “Siemens 2014,” a company-wide program aimed at improving profitability in the Sectors. One condition required for reaching the program’s ambitious target margin was a return to moderate revenue growth in fiscal 2014. This growth is not expected to materialize, due mainly to the market environment. As a result, Siemens no longer expects to achieve a Total Sectors profit margin of at least 12% by fiscal 2014. Measures for optimizing Siemens’ portfolio and reducing costs are largely on track.

Cost reduction measures in the Sectors focused primarily on improving regional footprints, adjusting capacity, and increasing process efficiency. These measures resulted in charges of €436 million overall, taken

4

Table of Contents

primarily in Infrastructure & Cities (€180 million), Industry (€140 million), and Energy (€102 million). Healthcare, which launched its productivity improvement measures a year ahead of “Siemens 2014,” recorded €14 million in related charges. Siemens expects substantial additional charges for “Siemens 2014” in the fourth quarter of the fiscal year.

Total Sectors profit declines on “Siemens 2014” charges. Total Sectors profit was €1.261 billion in the third quarter, down from the prior-year level due primarily to the “Siemens 2014” charges mentioned above. While Healthcare increased its profit year-over-year to €499 million, the other three Sectors reported lower profit compared to the prior-year period. Energy recorded a €91 million profit impact related to inspecting and retrofitting onshore wind turbine blades, and reported profit of €430 million. Industry’s profit declined to €347 million, and Infrastructure & Cities posted a loss of €15 million due to “Siemens 2014” charges.

Positive effect from progress with portfolio optimization. Income from continuing operations came in at €1.004 billion, down from €1.152 billion a year earlier. Corresponding basic EPS was €1.16 in the current period compared to €1.28 a year earlier. The main factor in the decline was lower Total Sectors profit. This was partly offset by improved results outside the Sectors. In particular, Equity Investments included a positive €301 million stemming from a partial reversal of a fiscal 2009 impairment on our stake in Nokia Siemens Networks B.V. (NSN). In addition, the equity investment loss related to Siemens’ stake in NSN narrowed year-over-year.

Net income rises on income from discontinued operations. Net income for the third quarter increased to €1.098 billion from €770 million a year earlier. Corresponding basic EPS rose to €1.27 from €0.85 in the prior-year period. These increases were due primarily to discontinued operations, which recorded income of €94 million compared to a loss of €382 million a year earlier. The primary factor in this improvement was a positive contribution of €42 million from OSRAM, compared to a negative €354 million in the third quarter a year earlier. That prior-year period included a negative catch-up effect of €443 million (pretax), arising when Siemens deemed it no longer highly probable to complete its original plan to dispose of OSRAM via an initial public offering. After the close of the third quarter, Siemens completed the spin-off and listing of OSRAM. As of June 30, 2013, Siemens recognized a spin-off liability amounting to €2.2 billion, reflecting 80.5% of the fair value of OSRAM. For further information regarding OSRAM, see Notes 2 and 18 in Notes to Condensed Interim Consolidated Financial Statements. Income from discontinued operations related to Siemens IT Solutions and Services was a positive €47 million in the current period compared to a negative €10 million in the same period a year ago.

5

Table of Contents

Increase in operating net working capital holds back Free cash flow. Free cash flow from continuing operations increased to €973 million from €899 million in the same period a year ago. Free cash flow was held back by an increase in operating net working capital of €1.3 billion, in part due to outstanding customer payments in the project business particularly in Energy and Infrastructure & Cities.

ROCE declines on lower income from continuing operations. On a continuing basis, ROCE (adjusted) decreased to 13.1%, compared to 14.5% a year earlier. The difference was due to lower income from continuing operations, while average capital employed was nearly unchanged year-over-year.

Decrease in pension plan underfunding. The estimated underfunding of Siemens’ pension plans as of June 30, 2013 amounted to €8.5 billion, compared to an estimated underfunding of €9.0 billion at the end of the second quarter. Siemens’ defined benefit obligation (DBO) decreased in the third quarter due to an increase in the discount rate assumption as of June 30, 2013.

RESULTS OF SIEMENS FOR THE NINE MONTHS ENDED JUNE 30, 2013

Orders and revenue

In the first nine months, Siemens won major long-cycle contracts for trains and offshore wind farms that drove a 12% increase in orders year-over-year. In contrast, revenue came in 2% lower compared to the prior-year period. The book-to-bill ratio for Siemens was 1.12, and the order backlog increased to €102 billion.

Orders related to external customers increased 12% compared to the prior-year period. The Infrastructure & Cities and Energy Sectors both won a number of major orders, including a contract win worth €3.0 billion for trains and maintenance in the U.K., which drove their double-digit order increases compared to the prior-year period. Healthcare showed slight order growth year-over-year with increases in most of its businesses. The market environment for Industry in the first nine months of fiscal 2013 was clearly more challenging than a year earlier and as a result orders for the Sector showed a clear and broad-based decline.

In the region comprising Europe, C.I.S., Africa, and the Middle East, nine-month orders increased substantially driven by double-digit increases in Energy and Infrastructure & Cities, due to higher volumes from large orders. Orders were stable year-over-year in the Americas, where a double-digit increase in Energy was offset by declines in the other Sectors. In the region Asia, Australia, Infrastructure & Cities showed a double- digit increase and Healthcare orders increased moderately compared to the first nine months a year ago. This growth was more than offset by a double-digit order decline in Energy and a moderate decline in Industry in the region. Emerging markets on a global basis grew more slowly than orders overall, at 8% year-over-year, and accounted for €20.925 billion, or 34%, of total orders for the first nine months of fiscal 2013.

6

Table of Contents

Revenue related to external customers declined 2% compared to the prior-year nine-month period, due in part to weaker investment sentiment in recent quarters. Slight increases in Infrastructure & Cities and Healthcare were more than offset by moderate declines in Energy and Industry.

Revenue declined slightly in the region Europe, C.I.S., Africa, Middle East, where a moderate increase at Infrastructure & Cities was more than offset by declines in the other Sectors. In the Americas revenue was down moderately, on declines in all Sectors. Asia, Australia also showed a slight decline in revenue, as clear growth in Healthcare was more than offset by declines in Industry and Energy. Emerging markets on a global basis were up 1% year-over-year, and accounted for €18.363 billion, or 33%, of total revenue for the first nine months.

Consolidated Statements of Income

Income from continuing operations before income taxes for the first nine months declined to €4.380 billion from €4.911 billion a year earlier, due primarily to €593 million in charges in the Sectors for the “Siemens 2014” program. These charges resulted from measures taken in the current period to reduce costs by

7

Table of Contents

improving regional footprints, adjusting capacity, and increasing process efficiency. All Sectors took a portion of these charges. They were recognized primarily in cost of goods sold (and, accordingly, in gross profit) and in marketing, selling and general administrative expenses which as a result increased compared to the prior-year period. In addition to the “Siemens 2014” program charges, gross profit was held back by lower capacity utilization in Industry as well as pricing pressure and a less favorable business mix in a number of Siemens businesses. In contrast, project charges were lower year-over-year, with a substantial decline in Energy more than offsetting an increase in Infrastructure & Cities.

Income from investments accounted for using the equity method, net was €352 million in the first nine months of fiscal 2013, compared to a loss of €391 million in the same period a year earlier. The major factor in this positive swing was the equity investment result related to our stake in Nokia Siemens Networks B.V. (NSN), which was a loss of €76 million in the current nine months compared to a loss of €768 million in the same period a year earlier. The change was due in part to a positive effect of €301 million in the current period, stemming from a partial reversal of a fiscal 2009 impairment of our stake in NSN. Improved results related to NSN were partly offset by a loss of €93 million related to our share in Enterprise Networks Holding B.V. (EN). The loss was due largely to additions to Siemens’ net investment in EN, which resulted in the recognition of previously unrecognized losses. The prior-year period benefited from a gain of €78 million on the sale of a portion of Financial Services’ (SFS) stake in Bangalore International Airport Limited.

Other financial income (expense), net was a negative €103 million, compared to a positive €87 million in the first nine months a year earlier. The current period included impairments of investments in Healthcare and SFS. The same period a year earlier included a gain of €87 million from the sale of our interest in OAO Power Machines.

The effective tax rate was 29% in the current reporting period, compared to 30% in the same period a year earlier.

As a result of the developments described above, Income from continuing operations decreased 8%.

Income from discontinued operations, net of income taxes in the first nine months of fiscal 2013 was €210 million, compared to a loss of €326 million in the first nine months of fiscal 2012. This positive swing was due mainly to OSRAM, which posted a profit of €178 million in the current period, compared to a loss of €218 million in the first nine months of fiscal 2012. The prior-year amount for OSRAM included a negative catch-up effect of €443 million (pretax), arising when we deemed it no longer highly probable to complete our original plan to dispose of OSRAM via an initial public offering. We subsequently completed the spinoff and listing of OSRAM according to a new plan, in the fourth quarter of fiscal 2013.

In addition, the loss from discontinued operations, net of income taxes in the first nine months of fiscal 2012 included a burden of €142 million (pretax) from a settlement related to Greece. For additional information on discontinued operations, see Note 2 in Notes to Condensed Interim Consolidated Financial Statements.

As a result, nine-month net income for Siemens was 8% higher than in the same period a year earlier. Net income attributable to shareholders of Siemens AG increased to €3.277 billion.

Portfolio activities

At the beginning of January 2013, Siemens acquired all of the shares in LMS International NV, Belgium (LMS), a leading provider of mechatronic simulation solutions. With the acquisition, which is being integrated in the Industry Sector’s Industry Automation Division, Siemens intends to expand and complement the Industry Sector’s product lifecycle management portfolio with mechatronic simulation and testing software. The preliminary purchase price amounts to €702 million (including €32 million cash acquired).

At the beginning of May 2013, Siemens acquired all the shares of six entities constituting the rail automation business of Invensys plc., U.K. (Invensys Rail). With the acquisition, which is being integrated in the Infrastructure & Cities Sector’s Mobility and Logistics Division, Siemens intends to expand and complement the Infrastructure & Cities Sector’s rail automation business. The preliminary purchase price amounts to €2.037 billion (including €57 million cash acquired) of which €472 million was paid to the Invensys Pension Trust.

8

Table of Contents

At the beginning of the fourth quarter of fiscal 2013, Siemens successfully completed the spin-off and listing of OSRAM and also announced the planned sale of its share in NSN.

For further information on portfolio activities, see Equity Investments and Subsequent events in the Interim Group Management Report and Note 2 in Notes to Condensed Interim Consolidated Financial Statements.

SEGMENT INFORMATION ANALYSIS FOR THE NINE MONTHS ENDED JUNE 30, 2013

Energy Sector

Energy reported a profit of €1.392 billion for the first nine months of fiscal 2013, down 20% year-over-year. The Sector took €149 million in charges under the “Siemens 2014” program, primarily for reducing the Sector’s cost structure, adjusting capacity and optimizing its regional footprint. Profit was also held back by €91 million in charges in the Wind Power Division related to inspecting and retrofitting onshore turbine blades mainly in the U.S. Fossil Power Generation contributed lower earnings than a year earlier, but still accounted for most of the Sector’s profit and was the highest profit performer among all Siemens Divisions. Profit at Oil & Gas declined due mainly to charges related to Iran, and Wind Power’s profit was also lower year-over-year, due mainly to the burden mentioned above. Power Transmission narrowed its loss compared to the prior-year period, due mainly to substantially lower project charges year-over-year. The solar business, which was reclassified to continuing operations in the second quarter and is reported within Energy on a retrospective basis, posted a loss of €225 million compared to a loss of €76 million in the same period a year earlier.

Revenue declined 4% compared to the prior-year period on decreases in all Divisions and all three reporting regions, while orders came in 16% higher due mainly to large orders at Wind Power. Order intake increased substantially in Europe, C.I.S., Africa, Middle East, due mainly to the higher volume from large orders at Wind Power. Order intake was significantly higher in the Americas, while orders declined substantially in the Asia, Australia region. The book-to-bill ratio for Energy was 1.10, and its order backlog was €55 billion at the end of the period.

9

Table of Contents

Fossil Power Generation generated profit of €1.305 billion in the first nine months of fiscal 2013, significantly below the profit of €1.557 billion in the first nine months of fiscal 2012. The Division recorded €67 million in charges for the “Siemens 2014” program. Profit development was also held back by a decline in revenue, a lower contribution from the service business, and a less favorable revenue mix in the products business. Reported profit in the first nine months of fiscal 2012 included an €87 million gain on the Division’s divestment of its joint venture stake in OAO Power Machines. Revenue was 9% lower year-over-year, resulting mainly from declining order intake for turnkey projects in prior quarters. On a geographic basis, revenue declined significantly in the Europe, C.I.S., Africa, Middle East region. Order intake was up 1%, as increases in the Americas and Europe, C.I.S., Africa, Middle East regions were almost offset by a substantial decrease in Asia, Australia.

Profit at Wind Power of €126 million in the first nine months of fiscal 2013 was substantially lower than in the same period a year earlier. Both periods included burdens on profit. In the current period, the Division took €91 million in charges related to inspecting and retrofitting onshore turbine blades mainly in the U.S. A year earlier, profit was held back by a €32 million provision related to a wind turbine component from an external supplier and a charge of €20 million related to capacity adjustment. Revenue was close to the prior-year level as a sharp decline in the Americas was almost offset by increases in other regions. Order intake almost doubled year-over-year, due mainly to a much higher volume from large orders, which included several large offshore wind-farms in Europe, C.I.S., Africa, Middle East.

Nine-month profit at Oil & Gas declined year-over-year, to €282 million. A more favorable revenue mix was more than offset by €46 million in charges in the first quarter resulting from compliance with newly enacted sanctions on Iran, primarily on its oil and gas industries, and €25 million in charges for the “Siemens 2014” program. Revenue was slightly lower compared to the same period a year earlier. Order intake was up 8% as increases in Europe, C.I.S., Africa, Middle East and the Americas more than offset a decline in Asia, Australia.

Power Transmission sharply reduced its nine-month loss compared to a year earlier, to €114 million. The Division took €134 million in project charges related mainly to grid connections to offshore wind-farms, compared to €503 million in charges in the prior-year period. With respect to two of these projects, material milestones are expected to be reached in the fourth quarter of fiscal 2013, including the technically highly complex and challenging transport and installation of these platforms for grid connections. In addition, it recorded €51 million in charges for the “Siemens 2014” program. Furthermore, profit development in the current period was held back by margin impacts related to these projects and by conversion of orders booked in prior periods with significant pricing pressure. In addition, operational challenges strongly cut back profit in the transformers and high-voltage products businesses. The prior-year period benefited from the release of a provision of €64 million related to successful project completion. Current nine-month revenue was down 3% year-over-year as declines in Europe, C.I.S., Africa, Middle East and Asia, Australia were partly offset by an increase in the Americas. Orders came in 2% lower compared to the prior-year period, due in part to more selective order intake in Europe, C.I.S., Africa, Middle East. This was partly offset by order increases in other regions. The Division expects continuing challenges in coming quarters.

10

Table of Contents

Healthcare Sector

For the first nine months of fiscal 2013, profit in the Healthcare Sector rose to €1.447 billion, led by earnings performances in its imaging and therapy systems businesses. Profit development benefited from lower charges associated with the Sector’s ongoing Agenda 2013 initiative which declined to €35 million from €144 million in the prior-year period. Measures associated with this initiative resulted in an improved cost position. The current period was burdened by a €36 million impairment of an investment at Diagnostics in Italy. Effective January 1, 2013, results for Healthcare include an excise tax on medical devices which was introduced in the U.S., and affects most businesses in the Sector. For comparison, the prior-year period benefited from the successful pursuit of a patent infringement claim of €34 million.

Diagnostics contributed €268 million to Sector profit, up significantly from €227 million in the prior-year period. Profit development followed the pattern for the Sector with regard to Agenda 2013, including lower charges and improvements in cost position. In particular, the charges fell to €13 million from €66 million in the prior-year period. The current period included the €36 million profit burden mentioned above for the Sector. Purchase price allocation (PPA) effects related to past acquisitions at Diagnostics were €128 million in the first nine months. A year earlier, Diagnostics recorded €129 million in PPA effects.

Healthcare revenue and orders remained stable year-over-year. On a regional basis, clear revenue growth from Asia, Australia was largely offset by lower revenue in the other regions. Orders grew moderately in Asia, Australia but declined in the Americas. The book-to-bill ratio was 1.00, and Healthcare’s order backlog was €7 billion at the end of the first nine months.

Revenue at Diagnostics was nearly unchanged year-over year, at €2.916 billion in the current period compared to €2.914 billion in the prior-year period. Diagnostics showed the same development as the Sector with regard to the regions.

Industry Sector

While the market environment for Industry in the first nine months of fiscal 2013 was clearly more challenging than a year earlier, the Sector saw signs of stabilizing at the end of the current period. This was particularly evident for the Sector’s short-cycle businesses. Due mainly to lower capacity utilization and a less favorable business mix, particularly in the first half of the current fiscal year, profit at Industry declined to €1.196 billion. Another major factor in the decline was €197 million in charges in the current period for the “Siemens 2014” program primarily to reduce costs associated with administrative processes and improve the Sector’s global footprint.

Revenue and orders for Industry in the first nine months were down 4% and 6%, respectively, including declines in both Divisions and the metals technologies business. On a geographic basis, revenue and orders declined in all three reporting regions, including double-digit declines in orders in the Americas. The Sector’s book-to-bill ratio was 1.00 and its order backlog at the end of the first nine months of fiscal 2013 was €11 billion.

11

Table of Contents