Sinclair Oil Corporation was an American petroleum corporation founded by Harry F. Sinclair on May 1, 1916. The Sinclair Oil and Refining Corporation amalgamated the assets of 11 small petroleum companies. Originally a New York corporation, Sinclair Oil reincorporated in Wyoming in 1976. The corporate logo featured the silhouette of a large green Brontosaurus dinosaur, based on the then-common idea that oil deposits beneath the earth came from the dead bodies of dinosaurs.

Sinclair was ranked as one of the largest privately owned American corporations. It owned and operated refineries, gas stations, hotels, a ski resort, and a cattle ranch.

Sinclair has long been a fixture on American roads with its dinosaur logo and mascot, a Brontosaurus.

At the Chicago World's Fair of 1933–1934, Sinclair sponsored a dinosaur exhibit meant to play on the link between the formation of petroleum deposits and the time of dinosaurs, now a largely discredited misconception. The exhibit included a 2-ton animated model of a Brontosaurus. The exhibit proved so popular it inspired a promotional line of rubber brontosaurs at Sinclair stations, complete with wiggling heads and tails, and the eventual inclusion of the brontosaur logo. Later, inflatable dinosaurs were given as promotional items. An anthropomorphic version appeared as a service-station attendant in advertisements. Some locations have a life-size model of the mascot straddling the building's entrance.

During September 1919, Harry Sinclair restructured Sinclair Oil and Refining Corporation, Sinclair Gulf Corporation, and 26 other related entities into Sinclair Consolidated Oil Corporation. In 1932, this new entity was renamed Consolidated Oil Corporation. In 1943, it was renamed Sinclair Oil Corporation. Near the beginning of the Great Depression, Sinclair sold the remaining interest in its pipeline subsidiary to Standard Oil Company (Indiana) for US$72.5 million (Standard Oil had purchased a 50% interest in the pipeline subsidiary in 1921). With these funds, including an additional US$33.5 million from an additional common stock issue, Sinclair retired several promissory notes and prepared to weather the Depression with the remaining supply of cash.

Between 1921 and 1922, Sinclair leased oil production rights to Teapot Dome in Wyoming without competitive bidding. This led to the Teapot Dome scandal. At the same time, Sinclair Oil was approached by the Italian fascist government. Benito Mussolini's government wanted to increase competition in the Italian oil market, which was controlled by the Italo-American Petroleum Society (SIAP), which in turn was fully dominated by Standard Oil. As the Teapot Dome scandal unfolded in the United States and reached the international press, Mussolini accelerated the negotiations, with a deal signed on May 4, 1924 (although without an official meeting, to avoid public outcry). Because of this, Sinclair Oil Company is known for having made "large payments to leading Fascists — all acting as intermediaries for Benito Mussolini — in return for an exclusive monopoly to drill for oil on Italian soil and in the Italian colonies". The deal was reported in a press release by the Head of Government (Mussolini) issued on the night of May 15, 1924, and published by most newspapers on the following day. The press release assured the public that Sinclair Oil had been awarded its contract on a competitive basis and had provided guarantees it had no relations with the international oil trust. This case of corruption was reported by the anti-fascist politician Giacomo Matteotti - who was later kidnapped and killed by Mussolini's newborn secret police, just before he could report his discoveries to the Parliament — in his posthumous article, published in the July issue of English Life (a magazine founded by Brendan Bracken): Matteotti accused Sinclair Oil of being a pawn of Standard Oil, as well as revealing "grave irregularities concerning the concession." Matteotti's theses were echoed in the notes of Epifanio Pennetta, who contributed to the preliminary investigation on the murder: "To all appearances," companies like Nafta and Saper "were in competition with the Sinclair company, while in fact they were in cahoots with Sinclair" and added that Sinclair Oil was actually working "in concert" with Standard Oil.

During the Great Depression, Sinclair saved many other petroleum companies from receivership or bankruptcy and acquired others to expand its operations. In 1932, Sinclair purchased the assets of Prairie Oil and Gas' pipeline and producing companies in the southern United States, and the Rio Grande Oil Company in California. The purchase of Prairie also gave Sinclair a 65% interest in Producers and Refiners Corporation (or Parco), which Sinclair subsequently acquired when Parco entered receivership in 1934. Lastly, in 1936, Sinclair purchased the East Coast marketing subsidiary of Richfield Oil Company, which had operated in receivership for several years. Richfield then reorganized, resulting in the creation of the Richfield Oil Corporation. Sinclair was instrumental in transferring capital and managerial assets into Richfield. Thirty years later, Richfield merged with Atlantic Refining, located on the East Coast, forming Atlantic Richfield.

In 1955, Sinclair ranked 21st on the Fortune 500; by 1969, it had fallen to 58th. In the early 1960s, Sinclair, along with Henry W. Peters and his son Eric Woods, developed the Turbo-S aircraft oils used for reliability in commercial jets, military jets, guided missiles and space exploration rockets. At the New York World's Fair of 1964–1965, Sinclair again sponsored a dinosaur exhibit, "Dinoland", featuring life-size replicas of nine different dinosaurs, including their signature Brontosaurus. Souvenirs from the exhibit included a brochure ("Sinclair and the Exciting World of Dinosaurs") and featured molded plastic dinosaur figurines. After the Fair closed, Dinoland remained as a traveling exhibit.