American petroleum corporation created in 1966 by the merger of Richfield Oil Corporation and Atlantic Refining Company. A further merger in 1969 brought in Sinclair Oil Corporation. Atlantic Richfield has petroleum operations in all parts of the United States as well as in Indonesia, the North Sea, and the South China Sea. The company also owns and operates transportation facilities for liquid petroleum, including pipelines and tankers; produces and sells chemicals, coal, and metal products; and is involved in the development of solar energy products and in other energy-related activities. Headquarters are in Los Angeles.

In 1977 the Anaconda Company—a miner and processor of copper, aluminum, and uranium and a manufacturer of copper and aluminum products—became a wholly owned subsidiary of Atlantic Richfield, although as a result of this merger Anaconda was forced to divest some of its copper interests. In 1981 Anaconda was fully merged into Atlantic Richfield.

Atlantic Refining, whose predecessor firms date back to the 1850s, was incorporated in 1870 and, after 1892, became one of the eastern companies of the Standard Oil Trust; after the U.S. Supreme Court's dissolution of the Standard Oil group in 1911, Atlantic Refining again became independent. Richfield, the product of several mergers in the first two decades of the 20th century, formally began in 1911 as a refining company financed jointly by the Los Angeles Oil and Refining Company and the Kellogg Oil Company (two of the several companies that were to merge under the Richfield name). Sinclair was formed in Kansas in 1916 by Harry F. Sinclair (1876–1956), who had made his first big oil strike in Oklahoma in 1907.