Martinez is the county seat of Contra Costa County, California, United States. The population was 35,866 at the 2000 census. The downtown is notable for its large number of preserved old buildings. Martinez is located on the south side of the Carquinez Strait in the San Francisco Bay Area, directly facing the city of Benicia. Martinez apartments for rent comparison page shows most up to date rent information on Martinez apartments and houses. Rent information is obtained from multiple rental sources but we cannot guarantee their accuracy.