Contra Costa County (Spanish for "opposite coast ) is a primarily suburban county in the San Francisco Bay Area of the U.S. state of California. As of the 2006, the US Census Bureau estimated it had a population of 1,024,319.

The county seat is Martinez. In prehistoric times, particularly the Miocene epoch, portions of the landforms now in the area (then marshy and grassy savanna) were populated by a wide range of now extinct mammals, known in modern times by the fossil remains excavated in the southern part of the county. In the northern part of the county, significant coal and sand deposits were formed in even earlier geologic eras. Other areas of the county have ridges exposing ancient but intact (not fossilized) seashells, embedded in sandstone layers alternating with limestone.