Rancho San Ysidro was a 13,066-acre (52.88 km2) Spanish land grant in present-day Santa Clara County, California, given in 1809 by Governor José Joaquín de Arrillaga to Ygnacio Ortega.
After Ygnacio's death in 1833, with Alta California under Mexican rule, Governor José Figueroa divided Rancho Ysidro among his three children (and their spouses).
Captain John C. Frémont and his troops passed through Rancho San Ysidro on their way south in November 1846 during the Mexican American War.
With the completion of the railroad and the demise of John Gilroy, the growth of the community of San Ysidro shifted two miles west.
The new Town of Gilroy was officially incorporated in 1870, becoming the largest community in Santa Clara County with a brewery, flouring mill and a distillery.