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.

Geographically divided by narrow points in the San Geronimo Valley, each has its own small commercial center.

[5] According to the U.S. Census Bureau, the CDP has a total area of 11.0 square kilometers (4.2 sq mi), all of it land.

Paper Mill Creek), one of the few remaining spawning grounds for Coho salmon (Oncorhynchus kisutch), runs through both towns.

These Coho are part of the Central California Coast Evolutionarily Significant Unit (CCC ESU), and are listed as "endangered" at both the state and federal level.

The towns line the western end of the San Geronimo Valley, extending into the forested south ridges and slightly into the grassier northern ones.

Lagunitas is on the eastern border of Samuel P. Taylor State Park.

Like the park, Lagunitas-Forest Knolls and the surrounding environs are lushly vegetated with large areas of coniferous forests.

There were 897 housing units at an average density of 211.2 per square mile (81.5/km2), of which 66.7% were owner-occupied and 33.3% were occupied by renters.

The racial makeup of the CDP in 2010 was 87.0% non-Hispanic White, 1.4% non-Hispanic African American, 0.2% Native American, 0.5% Asian, 0.1% Pacific Islander, 0.5% from other races, and 3.0% from two or more races.