Rancho Monserate was a 13,323-acre (53.92 km2) Mexican land grant in present-day San Diego County, California, given in 1846 by Governor Pío Pico to Ysidro María Alvarado.[1] The grant extended south and east of the present day Fallbrook down to the San Luis Rey River. The grant was bounded on the west by Pico's Rancho Santa Margarita y Las Flores.[2]