Lagunitas-Forest Knolls is a census-designated place, composed of two unincorporated areas in the western half of the San Geronimo Valley in Marin County, California. It is located to the west of San Geronimo and Woodacre. The population was 1,924 at the 2020 census. The two towns are locally seen as separate. Geographically divided by narrow points in the San Geronimo Valley, each has its own small commercial center. Both towns are primarily residential.