Mare Island is a peninsula in the United States alongside the city of Vallejo, California, about northeast of San Francisco. The Napa River forms its eastern side as it enters the Carquinez Strait juncture with the east side of San Pablo Bay. Mare Island is considered a peninsula because no full body of water separates this or several other named "islands" from the mainland. Instead, a series of small sloughs cause seasonal water-flows among the so-called islands. Mare Island is the largest of these at about long and a mile wide.
The Napa River widens and forms a harbor between Mare Island and the mainland.