San Jacinto is a city in Riverside County, California. It was named after Saint Hyacinth and is located at the north end of the San Jacinto Valley, with Hemet to its south and Beaumont, CA to its north. The mountains associated with the valley are the San Jacinto Mountains. The population was 44,199 at the 2010 census. The city was founded in 1870 and incorporated on April 20, 1888, making it one of the oldest cities in Riverside County.The city is home to Mt.