The Cowell Ranch/John Marsh State Historic Park is an upcoming state park in Brentwood, California. It will offer 3659 acres of area composed of various natural habitats.

Best noted for its historic significance, the Cowell Ranch/John Marsh State Historic Park is also one of the newest acquisitions of the California State Parks system. It will feature the former home of John Marsh, who is the very first American to settle in what is known today as the Contra Costa County. He lived on the property from 1837 to 1856.

Through the joint efforts of the Trust for Public Land and some concerned citizens with the help of other entities, the Cowell Ranch/John Marsh State Historic Park was acquired by the state and is now being developed to serve as a new state park.