Rancho Saucelito (also called "Rancho Sausalito") was a 19,752-acre (79.93 km2) Mexican land grant in present-day Marin County, California, given in 1838 by Governor Juan Alvarado to William A. Richardson. The name means "ranch of the little willow grove". The grant extended from the Pacific Ocean on the west, to Mount Tamalpais to the north, and the Arroyo Corte Madera del Presidio and Richardson Bay on the east; and included present-day Muir Beach, Stinson Beach, Sausalito, Tamalpais Valley, and Homestead Valley.
The grant was originally made to José Antonio Galindo in 1835. José Antonio Galindo also received Rancho Laguna de la Merced in 1835. In 1838, José Antonio Galindo was arrested for the murder of José Doroteo Peralta (1810 - 1838), son of Pedro Peralta. Rancho Saucelito was re-granted to William A. (Guillermo Antonio) Richardson.
With the cession of California to the United States following the Mexican-American War, the 1848 Treaty of Guadalupe Hidalgo provided that the land grants would be honored. As required by the Land Act of 1851, a claim for Rancho Saucelito was filed with the Public Land Commission in 1852, and the grant was patented to William Richardson in 1879.