Tomales Bay is a long, narrow inlet of the Pacific Ocean in Marin County in northern California in the United States.

[6] Watercrafts may be launched on Tomales Bay from the public boat ramp at Nick's Cove, north of Marshall.

The California Office of Environmental Health Hazard Assessment (OEHHA) has developed a safe eating advisory for fish caught here, based on levels of mercury or PCBs found in local species.

In the bay’s waters, bony and cartilaginous fish species including halibut, coho salmon, bat rays and leopard sharks[9] can be found.

[10] Along muddy parts of bay's shore, it is common to find the gastropods such as the invasive False Cerith snail, recognizable from its dextrally coiled shape and brown-gray pattern.

[14] Members of the Vizcaíno Expedition found the Bay in 1603, and thinking it a river, named it Rio Grande de San Sebastian.

[15] Early 19th-century settlements constituted the southernmost Russian colony in North America and were spread over an area stretching from Point Arena to Tomales Bay.

[16] The narrow gauge North Pacific Coast Railroad from Sausalito was constructed along the east side of the bay in 1874 and extended to the Russian River until it was dismantled in 1930.