Rancho Boca de Santa Monica (mouth of Santa Monica) was a 6,656-acre (26.94 km2) Mexican land grant in present day Los Angeles County, California given by governor Juan Alvarado in 1839 to Ysidro Reyes and Francisco Marquez.
In 1839, Ysidro Reyes (1813–1861) and Francisco Marquez (1798–1850) were granted Rancho Boca de Santa Monica, comprising what is now Santa Monica Canyon, the Pacific Palisades, and parts of Topanga Canyon.
Francisco Marquez and his wife, Roque Valenzuela, built an adobe house in the upper mesa of the canyon. Marquez built a blacksmith shop and continued to live and work the rancho until his death in 1850.
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 Boca de Santa Monica was filed with the Public Land Commission in 1852. The boundary with Rancho San Vicente y Santa Monica was disputed, and was not finally resolved of until 1882, when the United States courts patented Rancho Boca de Santa Monica to Marquez and Reyes at 6,656 acres (27 km2).
Ysidro Reyes and his wife, Maria Antonia Villa, built a house in what is now the Huntington Palisades. Ysidro Reyes died in 1861, leaving his undivided one-half interest in the rancho to his widow Maria Antonia. In 1872, Maria Antonia Reyes sold that interest to Col. Robert S. Baker. In 1874, Baker filed suit to partition the land among himself and the heirs of Francisco Marquez, who jointly held the other one-half interest.
In the mid-1920s, the families sold the land to Santa Monica Land & Water Co., owned by Robert Gillis.
Descendant Ernest Marquez has documented the Rancho Boca de Santa Monica since the 1950s.