Martinez (märtēˈnəs) [key], city (1990 pop. 31,808), seat of Contra Costa co., W Calif., on Carquinez Strait between San Pablo and Suisun bays, in a farm area; inc. 1884. Its major industry is petroleum refining. The home of the naturalist John Muir, which has been preserved as a national historic site, is in the city. A naval weapons station adjoins the city, and part of the Central Valley project is nearby.