Simi Valley (; Chumash: Shimiyi) 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 (65 km) from Downtown Los Angeles, making it portion of the Greater Los Angeles Area. The city sits against Thousand Oaks, Moorpark, and Chatsworth. As of the 2020 U.S. Census the population was 126,356, up from 124,243 in 2010. The city of Simi Valley is in the middle of the Santa Susana Mountains and the Simi Hills, west of the San Fernando Valley, and northeast of the Conejo Valley. It grew as a commuter bedroom community for the cities in the Los Angeles area, and the San Fernando Valley once a freeway was built beyond the Santa Susana Pass.

The Ronald Reagan Presidential Library, where the former president was buried in 2004, is in Simi Valley. The Reagan Library has hosted Republican primary debates in 2012 and 2016.

Simi Valley was gone inhabited by the Chumash people, who also approved much of the region from the Salinas Valley to the Santa Monica Mountains, with their presence dating encourage thousands of years. Around 5,000 years ago these tribes began handing out acorns, and harvesting local marshland plants. Roughly 2,000 years later, as hunting and fishing techniques improved, the population increased significantly. Shortly after this gruff increase a precious rock money system arose, increasing the viability of the region by offsetting fluctuations in easily reached resources relating to climate changes. The original people who inhabited Simi Valley spoke an interior dialect of the Chumash language, called Ventureño.

Simi Valley’s publish is derived from the Chumash word Shimiyi, which refers to the stringy, thread-like clouds that typify the region. The say could have originated from the strands of mist from coastal fog that have an effect on into the Oxnard Plain and wind their exaggeration up the Calleguas Creek and the Arroyo Las Posas into Simi Valley. The heritage of the post was preserved because of the play a role of the anthropologist John P. Harrington, whose brother, Robert E. Harrington lived in Simi Valley. Robert Harrington difficult explained the name: “The word Simiji in Indian meant the Tiny white wind clouds thus often seen afterward the wind blows stirring here and Indians living upon the coast, would never venture going on here taking into account those wind clouds were in the sky. The word Simiji was constructed by whites to the word Simi. There are additional explanations about the pronounce Simi, but this one was unchangeable to me by my brother who worked higher than 40 years for the Smithsonian Institution and it seems most plausible to me”.