Tomales Bay is a long, narrow inlet of the Pacific Ocean in Marin County in northern California in the United States. It is approximately 15 mi long and averages nearly 1.0 mi wide, effectively separating the Point Reyes Peninsula from the mainland of Marin County. It is located approximately 30 mi northwest of San Francisco. The bay forms the eastern boundary of Point Reyes National Seashore. Tomales Bay is recognized for protection by the California Bays and Estuaries Policy. On its northern end, it opens out onto Bodega Bay, which shelters it from the direct current of the Pacific. The bay is formed along a submerged portion of the San Andreas Fault.
Oyster farming is a major industry on the bay. The two largest producers are Hog Island Oyster Company and Tomales Bay Oyster Company, both of which retail oysters to the public and have picnic grounds on the east shore. Hillsides east of Tomales Bay are grazed by cows belonging to local dairies. There is also grazing land west of the bay, on farms and ranches leased from Point Reyes National Seashore.
The bay sees significant amounts of water sports including sailing, kayaking, fishing and motor boating.