Monte Rio is a census-designated place (CDP) in Sonoma County, California along the Russian River near the Pacific Ocean. The town of Guerneville lies northeast of Monte Rio, and Jenner is to the west. The population was 1,152 at the 2010 census, up from 1,104 at the 2000 census. Bohemian Grove is located in Monte Rio.

The Russian River is a southward-flowing river that drains 1,485 sq mi (3,850 km2) of Sonoma and Mendocino counties in Northern California. With an annual average discharge of approximately 1,600,000 acre feet (2.0 km3), it is the second-largest river flowing through the nine-county Greater San Francisco Bay Area, with a mainstem 115 mi (185 km) long.