Table of Contents

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

FORM 10-K

For The Fiscal Year Ended December 31, 2005

OR

Commission File Number 1-3610

ALCOA INC.

(Exact name of registrant as specified in its charter)

Registrant’s telephone numbers:

Securities registered pursuant to Section 12(b) of the Act:

Securities registered pursuant to Section 12(g) of the Act: None

Indicate by check mark if the registrant is a well-known seasoned issuer, as defined in Rule 405 of the Securities Act.    Yes  x    No  ¨.

Indicate by check mark if the registrant is not required to file reports pursuant to Section 13 or Section 15(d) of the Exchange Act.    Yes  ¨    No  x.

Indicate by check mark whether the registrant (1) has filed all reports required to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during the preceding 12 months, and (2) has been subject to such filing requirements for the past 90 days.    Yes  x    No  ¨.

Indicate by check mark if disclosure of delinquent filers pursuant to Item 405 of Regulation S-K is not contained herein, and will not be contained, to the best of registrant’s knowledge, in definitive proxy or information statements incorporated by reference in Part III of this Form 10-K or any amendment to this Form 10-K.  x

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, or a non-accelerated filer. See definition of “accelerated filer” and “large accelerated filer” in Rule 12b-2 of the Exchange Act.

Indicate by check mark whether the registrant is a shell company (as defined in Rule 12b-2 of the Exchange Act).    Yes  ¨    No  x.

The aggregate market value of the outstanding common stock, other than shares held by persons who may be deemed affiliates of the registrant, as of the last business day of the registrant’s most recently completed second fiscal quarter was approximately $23 billion. As of February 14, 2006, there were 871,819,586 shares of common stock, par value $1.00 per share, of the registrant outstanding.

Documents incorporated by reference.

Parts I, II and IV of this Form 10-K incorporate by reference certain information from the registrant’s 2005 Annual Report to Shareholders (Annual Report). Part III of this Form 10-K incorporates by reference certain information from the registrant’s definitive Proxy Statement for its 2006 Annual Meeting of Shareholders filed or to be filed pursuant to Regulation 14A (Proxy Statement).

Table of Contents

TABLE OF CONTENTS

Note on Incorporation by Reference

In this Form 10-K, selected items of information and data are incorporated by reference to portions of the Annual Report and Proxy Statement. Any reference in this report to disclosures in the Annual Report or Proxy Statement shall constitute incorporation by reference of that specific disclosure into this Form 10-K.

2

Table of Contents

ALCOA INC.

Formed in 1888, Alcoa Inc. is a Pennsylvania corporation with its principal office in New York, New York. In this report, unless the context otherwise requires, Alcoa or the “company” means Alcoa Inc. and all subsidiaries consolidated for the purposes of its financial statements.

The company’s Internet address is http://www.alcoa.com. Alcoa makes available free of charge on or through its website its annual report on Form 10-K, quarterly reports on Form 10-Q, current reports on Form 8-K, and amendments to those reports filed or furnished pursuant to Section 13(a) or 15(d) of the Securities Exchange Act of 1934 as soon as reasonably practicable after the company electronically files such material with, or furnishes it to, the Securities and Exchange Commission (SEC). The SEC maintains an Internet site that contains these reports at http://www.sec.gov.

PART I

Item 1. Business.

Description of the Business

Information describing Alcoa’s businesses can be found in the Annual Report at the indicated pages:

Overview

Alcoa is the world’s leading producer of primary aluminum, fabricated aluminum, and alumina, and is active in all major aspects of the industry: technology, mining, refining, smelting, fabricating, and recycling. Aluminum is a commodity that is traded on the London Metal Exchange (LME) and priced daily based on market supply and demand. Aluminum and alumina represent approximately three-quarters of Alcoa’s revenues, and the price of aluminum influences the operating results of Alcoa. Nonaluminum products include precision castings, industrial fasteners, vinyl siding, consumer products, food service and flexible packaging products, plastic

3

Table of Contents

closures and electrical distribution systems for cars and trucks. Alcoa’s products are used worldwide in aircraft, automobiles, commercial transportation, packaging, consumer products, building and construction, and industrial applications.

Alcoa is a global company operating in 42 countries. North America is the largest market with 61% of Alcoa’s revenues. Europe is also a significant market with 23% of the company’s revenues. Alcoa also has investments and activities in Australia, Brazil, China, Iceland, Jamaica, Russia, and Trinidad, which present opportunities for substantial growth. Governmental policies and other economic factors, including inflation and fluctuations in foreign currency exchange rates and interest rates, affect the results of operations in these countries.

Alcoa’s operations consist of six worldwide segments: Alumina, Primary Metals, Flat-Rolled Products, Extruded and End Products, Engineered Solutions, and Packaging and Consumer.

The Alumina segment primarily consists of a series of affiliated operating entities referred to as Alcoa World Alumina and Chemicals (AWAC). Generally, Alcoa owns 60% and Alumina Limited owns 40% of these entities. For more information on AWAC, see Exhibit Nos.10(a) through 10(e) to this report.

Bauxite Interests

Aluminum is one of the most plentiful elements in the earth’s crust. Aluminum is produced primarily from bauxite, an ore containing aluminum in the form of aluminum oxide, commonly referred to as alumina. Aluminum is made by extracting alumina from bauxite and then removing oxygen from the alumina. Alcoa processes most of the bauxite that it mines into alumina. The company obtains bauxite from reserves held by AWAC, from the company’s interests in the countries listed in the chart below, and under both long-term and short-term contracts and mining leases. In 2005, Alcoa consumed 31.5 million metric tons (mt) of bauxite from its own reserves, 7.1 million mt from related third parties and 2.5 million mt from unrelated third parties. Alcoa’s present sources of bauxite are sufficient to meet the forecasted requirements of its alumina refining operations for the foreseeable future. The following table provides information regarding the company’s bauxite interests:

Alcoa Active¹ Bauxite Interests

4

Table of Contents

Alumina Refining Facilities and Capacity

Alcoa is the world’s leading producer of alumina. Alcoa’s alumina refining facilities and its worldwide alumina capacity are shown in the following table:

Alcoa Worldwide Alumina Refining Capacity

5

Table of Contents

In November 2005, AWA LLC and Alcan announced the signing of a Basic Agreement with the Government of Guinea that sets forth the framework for development of a 1.5 million mtpy alumina refinery in Guinea with further expansion potential.

In September 2005, Alcoa announced that its Board of Directors approved plans to make further investments totaling $1.6 billion in the company’s Brazilian “upstream” operations, including: a 2.1 million mtpy expansion of the Alumar consortium alumina refinery in São Luís, state of Maranhao; the creation of a bauxite mine in Juruti, in the state of Para, which will initially produce 2.6 million mtpy; and the modernization of the Poços de Caldas aluminum smelter, in the state of Minas Gerais. Pending finalization of definitive documents, which is expected in due course, the Alumar expansion project will increase the refinery’s capacity from 1.4 million mtpy to approximately 3.5 million mtpy. Through AWAC, Alcoa will own a 54% interest in the expanded refinery and its share of the total facility output will more than double to 1.89 million mtpy.

In June 2005, Alcoa’s Board of Directors approved AWAC’s planned expansion of the Jamalco alumina refinery in Clarendon, Jamaica. Pending finalization of definitive documents, which is expected in due course, this project will add 1.5 million mtpy of production to Jamalco, more than doubling the refinery’s capacity to approximately 2.8 million mtpy. The 1.5 million mtpy expansion is expected to cost approximately $1.2 billion. As a result of this expansion project, AWAC’s ownership in Jamalco will increase from 50% to approximately 77%, with the Government of Jamaica continuing to own the remaining minority interest.

In January 2005, Alcoa and the Government of the Republic of Ghana announced the signing of a Memorandum of Understanding (MOU), under which the parties would evaluate the possible development of an integrated aluminum industry in Ghana, including bauxite mining, alumina refining, aluminum production, and rail transportation infrastructure upgrades. Alcoa is working with the Government to conduct feasibility studies that are expected to be completed in 2006, after which the parties will negotiate definitive agreements on the mining, refining, smelting, rail upgrades and ownership structure, as well as total investment costs. The feasibility review will include a study on the creation of an alumina refinery with an initial design capacity of up to 1.5 million mtpy.

6

Table of Contents

Primary Aluminum Facilities and Capacity

The company’s primary aluminum smelters and their respective capacities are shown in the following table:

Alcoa Worldwide Smelting Capacity

7

Table of Contents

Alcoa currently has 509,000 mtpy of idle capacity against a base capacity of 4,004,000 mtpy.

In February 2006, Alcoa signed an Agreement in Principle with the Government of the Republic of Trinidad and Tobago to build a 341,000 mtpy aluminum smelter in the Cap-de-Ville area in southwestern Trinidad. This agreement follows the signing of an MOU in May 2004 for participation by Alcoa in the development of an aluminum industry in Trinidad and Tobago. Under the Agreement in Principle, Alcoa will take a lead role in the development and operation of the smelter, with the Government installing necessary infrastructure. Alcoa will also sponsor development of a new power station to support the facility. Negotiations are ongoing for the final commercial arrangements, which will be subject to final approvals by Alcoa’s Board of Directors and by the Government.

During 2005, Aluminio began a 30% expansion of the capacity of its share of the São Luís (Alumar) aluminum smelter, increasing Aluminio’s share of smelting capacity there by 62,000 mtpy to a total of 263,000 mtpy. When complete, Alcoa’s share of output from the smelter will grow from 53.66% to 60%. Completion of the expansion and startup of the additional capacity is expected by the end of the first quarter of 2006.

As noted above, in September 2005, Alcoa announced that its Board of Directors approved plans to make further investments totaling $1.6 billion in the company’s Brazilian upstream operations, including the modernization of the Poços de Caldas aluminum smelter, in the state of Minas Gerais. The modernization is aimed at reducing emissions and costs and improving operational efficiency.

During 2005, Alcoa continued construction on its new 346,000 mtpy Fjarðaál aluminum smelter in east Iceland. Alcoa broke ground on the new smelter in July 2004. The smelter is scheduled to begin production in April 2007.

Alcoa has a Joint Action Plan in place with the Government of Iceland and three communities in northern Iceland to evaluate the construction of an up to 250,000 mtpy smelter. Through the fall of 2005, three sites have been evaluated and a conclusion on whether to proceed further is expected by March 2006. An MOU covering further development of this smelter project based on a selected site is expected.

In 2005, Alcoa continued construction of a new anode plant in Mosjøen, Norway. The facility, being built together with Elkem AS, will produce anodes for Alcoa’s Fjarðaál, Iceland and Elkem Aluminium ANS’ Mosjøen, Norway smelters. Construction of the anode plant is expected to be completed by 2007.

Alcoa is exploring aluminum smelting and associated hydroelectric power opportunities in Suriname. In January 2003, Suralco, one of the AWAC group of companies, signed an MOU with the Government of Suriname providing for an initial 18-month exclusive period of investigation of the feasibility of smelting and associated hydroelectric power investment in western Suriname. In December 2004, the parties agreed, retroactively, to extend the MOU through December 31, 2005. Alcoa is in the process of negotiating a follow on agreement with the Government.

8

Table of Contents

Alcoa has an exclusive MOU in effect with the Brunei Economic Development Board (BEDB) for building a smelter in Brunei. In November 2005, the BEDB received a McKinsey report evaluating the use of Brunei’s gas reserves for downstream industry. The potential smelter is one of several industrial developments being considered by the BEDB.

Alcoa owns interests in the following primary aluminum facilities that are accounted for on the equity or cost basis method. The capacity associated with these facilities is not included in Alcoa’s consolidated capacity.

As noted above, in January 2005, Alcoa and the Government of the Republic of Ghana signed an MOU, under which the parties would evaluate the development of an integrated aluminum industry in Ghana, including aluminum production. Alcoa is working with the Government to conduct feasibility studies for the project. These studies are expected to be completed in 2006, at which time the parties would negotiate definitive agreements on smelting and other matters.

At the end of 2005, the shareholders of Hamburger Aluminium-Werk closed the 132,000 mtpy Hamburg, Germany smelter due to escalating power costs. The Hamburg smelter is owned 33.33% each by Alcoa, Austria Metall AG and Norsk Hydro.

Energy

Alcoa produces aluminum from alumina by an electrolytic process requiring large amounts of electric power. Electric power accounts for approximately 25% of the company’s primary aluminum costs. Alcoa generates approximately 24% of the power used at its smelters worldwide and generally purchases the remainder under long-term arrangements. The paragraphs below summarize the sources of power and long-term power arrangements for Alcoa’s smelters.

North America - Electricity

The company’s wholly-owned subsidiary, Alcoa Power Generating Inc. (APGI), generates approximately 25% of the power requirements for Alcoa’s North American smelters. The company generally purchases the remainder under long-term contracts. APGI owns and operates two hydroelectric projects, Tapoco and Yadkin, consisting of eight dams under Federal Energy Regulatory Commission (FERC) licenses. APGI hydroelectric facilities provide electric power for the aluminum smelters at Alcoa, Tennessee and Badin, North

9

Table of Contents

Carolina. APGI received a renewed 40-year FERC license for the Tapoco project in 2005 and the Yadkin hydroelectric project license is up for renewal in 2008. With the Badin smelter idled, power generated from APGI’s Yadkin system is largely being sold to an affiliate, Alcoa Power Marketing, Inc. The Tennessee smelter also purchases power from the Tennessee Valley Authority under a contract that extends to 2010.

In the Pacific Northwest, Alcoa has a contract with Chelan County Public Utility District located in the State of Washington that is sufficient to supply about half of the capacity of the Wenatchee smelter through October 2011. In addition, Alcoa has a contract through September 2006 for a limited amount of power from the Bonneville Power Administration (BPA) that can be used to cover the remainder of the capacity at the Wenatchee smelter or to operate the Intalco smelter. Alcoa is currently returning part of its BPA power through the end of the contract period. BPA has embarked on the contracting process for the period October 2006-September 2011, but it is not yet clear whether the BPA offer will be adequate for Intalco and/or Wenatchee operations.

The company, through APGI, generates substantially all of the power used at its Warrick smelter using nearby coal reserves. A 1996 coal supply contract satisfies up to 70% of the fuel requirements of the Warrick power plant supplying the smelter through June 2006. In May 2005, Alcoa acquired mining rights to the nearby Friendsville, Illinois coal reserves, and it expects coal from this mine to be available for its use by the summer of 2006. When fully operational, the mine will be capable of producing approximately 1 million tons of coal per year, approximately 45% of the Warrick power plant’s requirements. In April 2001, under the terms of an operating agreement, the company assumed operation of the power plant that supplies the Warrick smelter from Vectren (formerly Southern Indiana Gas & Electric Company) until at least 2008. In July 2005, Alcoa announced its plans to invest approximately $330 million at the Warrick power plant to improve environmental performance and operational efficiency and lower costs.

Power for the Rockdale smelter is generated by company-owned generating units and TXU Generation Company LP (TXU)-owned generating units. Historically, both the company-owned and the TXU-owned units used lignite supplied by the company’s Sandow Mine. The company has opened a new lignite mine, the Three Oaks Mine, on adjacent land it owns or controls. The fuel supply for the company-owned and TXU-owned units is transitioning from the Sandow Mine to the Three Oaks Mine, with active mining in Sandow expected to conclude in 2006. Company-owned generating units supply about one-half of the total electricity requirements of the smelter, although this capacity will retire not later than 2007. TXU supplies the balance through a long-term power contract that does not expire until at least the end of 2038, with the parties having the right to terminate the contract after 2013 if there has been an unfavorable change in law or after 2025 if the cost of the electricity exceeds the market price. TXU and Alcoa recently signed a letter of intent in connection with the possible development of a new circulating fluidized bed power plant adjacent to the existing Sandow Unit Four Power Plant. Alcoa would supply lignite for the new unit from the Three Oaks Mine, and have a right to purchase a portion of the plant’s electricity output.

In the Northeast, the purchased power contracts for both the Massena East and Massena West smelters in New York expire not earlier than June 30, 2013, following their extension for 10 years upon New York Power Authority (NYPA) having relicensed its St. Lawrence-FDR Hydro Project. The company is currently in discussions with NYPA to extend the power supply arrangements in order to facilitate potential technical improvements in the plants, but may terminate either of these contracts with one year’s notice.

The Deschambault and Bécancour smelters located in Quebec purchase electricity under long-term contracts with Hydro-Quebec that expire in 2014, subject to extension provisions. The smelter located in Baie Comeau, Quebec purchases approximately 65% of its power needs under the Hydro-Quebec contract and receives the remainder of its power needs from a 40%-owned hydroelectric generating company, Manicouagan Power Company.

The Mt. Holly smelter