San Geronimo (formerly, Nicasio) is a census-designated place (CDP) located in the San Geronimo Valley in Marin County, California in the United States. San Geronimo is located 8 miles (13 km) southwest of downtown Novato, at an elevation of 292 feet (89 m). The population was 446 at the 2010 census.
San Geronimo is located at 38°00′48″N 122°39′50″W.
According to the United States Census Bureau, the CDP has a total area of 1.5 square miles (3.9 km2), all of it land.
Prior to 1877, the place was called Nicasio. The San Geronimo post office opened in 1895, closed in 1910, and re-opened in 1911.