The General Motors streetcar conspiracy refers to the convictions of General Motors (GM) and related companies that were involved in the monopolizing of the sale of buses and supplies to National City Lines (NCL) and subsidiaries, as well as to the allegations that the defendants conspired to own or control transit systems, in violation of Section 1 of the Sherman Antitrust Act. This suit created lingering suspicions that the defendants had in fact plotted to dismantle streetcar systems in many cities in the United States as an attempt to monopolize surface transportation.

National City Lines, Inc. (NCL) was a public transportation company. The company grew out of the Fitzgerald brothers' bus operations, founded in Minnesota, United States, in 1920 as a modest local transport company operating two buses. Part of the Fitzgerald's operations were reorganized into a holding company in 1936, and later expanded about 1938 with equity funding from General Motors, Firestone Tire, Standard Oil of California and Phillips Petroleum for the express purpose of acquiring local transit systems throughout the United States in what became known as the General Motors streetcar conspiracy. The company formed a subsidiary, Pacific City Lines in 1937 to purchase streetcar systems in the western United States. National City Lines, and Pacific City Lines were indicted in 1947 on charges of conspiring to acquire control of a number of transit companies, and of forming a transportation monopoly for the purpose of "conspiring to monopolize sales of buses and supplies to companies owned by National City Lines." They were acquitted on the first charge and convicted on the second in 1949.