Simi Valley is a city in the valley of the same name in the southeast region of Ventura County, California, United States. Simi Valley is 40 miles from Downtown Los Angeles, making it part of the Greater Los Angeles Area. The city borders Thousand Oaks, Moorpark, and the Chatsworth neighborhood of Los Angeles. As of the 2020 U.S. Census the population was 126,356, up from 124,243 in 2010.