Nicasio is a census-designated place in Marin County, California. It is located 8 miles (13 km) west-southwest of Novato, at an elevation of 194 feet (59 m). The population was 96 at the 2010 census.
Nicasio (ni-kash'-oh, or ni-cah'-see-oh) is a contiguous area of unincorporated land in west Marin County. The Nicasio region encompasses approximately 25,000 acres (100 km²) and has no legally defined political boundaries. Geographically it is a hydrologic zone containing the four main drainages of Nicasio Creek up to the ridgelines defining their basin. This includes: the south fork of Nicasio Creek extending from Moon Hill, along the ridge separating Nicasio from San Geronimo and Samuel P. Taylor parks, and extending northwest towards the northern end of Platform Bridge Road; the east fork of Nicasio Creek extending from Loma Alta and the ridge on the north side of Sir Francis Drake Boulevard, extending up through Big Rock and up towards Big-Rock Ridge; the Halleck Creek drainage, up to the ridges separating Nicasio from Lucas Valley and Novato; the northern drainages extending along the ridgelines of Rocky Ridge, Hicks Mountain and Black Mountain.
A major landmark in the area is the man-made Nicasio Reservoir. Nicasio is scenically very attractive with forested and open ridgelines surrounding a wide rolling valley. The area along Lucas Valley Road follows a narrow valley with mostly forested hillsides above it. Oak, bay, and fir trees are common in the area, and along some of the creeks stands of redwoods are still found. Wildlife is abundant. Three main roads cross the area: Lucas Valley Road, Nicasio Valley Road, and the Point Reyes-Petaluma Road. Important pieces of public land in the surrounding area include Point Reyes National Seashore, portions of the Golden Gate National Recreation Area, and Samuel P. Taylor State Park.