The Mexican oil expropriation (Spanish: expropiacin petrolera) was the nationalization of all petroleum reserves, facilities, and foreign oil companies in Mexico on March 18, 1938. In accordance with Article 27 of the Constitution of 1917, President Lzaro Crdenas declared that all mineral and oil reserves found within Mexico belong to "the nation", i.e., the federal government. The Mexican government established a state-owned petroleum company, Petrleos Mexicanos, or PEMEX. For a short period, this measure caused an international boycott of Mexican products in the following years, especially by the United States, the United Kingdom, and the Netherlands, but with the outbreak of World War II and the alliance between Mexico and the Allies, the disputes with private companies over compensation were resolved. The anniversary, March 18, is now a Mexican civic holiday.

Pemex (a portmanteau of Petróleos Mexicanos, which translates to Mexican Petroleum in English; Spanish pronunciation: [ˈpemeks]) is the Mexican state-owned petroleum company managed and operated by the Mexican government. It was formed in 1938 by nationalization and expropriation of all private oil companies in Mexico at the time of its formation. Pemex had total assets worth $101.8 billion in December 2019 and as of 2009 was Latin America's second largest enterprise by annual revenue, surpassed only by Petrobras (the Brazilian national oil company). The company is the 7th most polluting in the world according to The Guardian.