Contra Costa (Spanish for "opposite coast") is one of the 58 counties in the state of California. It was created in 1850, making it one of the original counties of the state. As of the 2010 census, the population was 1,049.025. The county seat is Martinez.
Contra Costa has an area of 802.15 sq mi (2,077.6 km2), of which 719.95 sq mi (1,864.7 km2) is land. The county is bordered on the south and west by Alameda County, and has bridge links to Marin County to the west, Napa, Solano and Sacramento counties to the north, and San Joaquin county to the east.
Contra Costa is one of the nine traditional Bay Area counties.