Mare Island is an island in Vallejo, California, about 30 miles northeast of San Francisco. The Napa River forms its eastern side, the Carquinez Strait its south end, and San Pablo Bay its western side. Some consider it to be part of a peninsula because on the north no full body of water separates this or several other named "islands" from the mainland. Instead, a series of small sloughs nearly separates several chunks of dry land from each other; in extremely wet weather, the sloughs can in fact make the chunks appear to be separate islands. Mare Island is the largest of these at about 3.5 miles long and a mile wide.

This area was part of Rancho Soscol, deeded to General Mariano Guadalupe Vallejo in 1844. According to the story, his favorite white mare fell off a raft while being transported across the Carquinez Straits and survived drowning by swimming to an island, which he named Isla de la Yegua (Mare Island) in her honor.

The Napa River widens and forms an excellent harbor between Mare Island and the mainland. The United States bought Mare Island in July, 1852, for use as a naval shipyard. Two years later, on September 16, 1854, Mare Island became the first permanent U.S. naval installation on the west coast.

Mare Island was used for more than a century as the United States Navy's Mare Island Naval Shipyard. Most of the naval shipyard was decommissioned in the 1990s and is now being developed for various public and private enterprises and activities.