Winchester is a census-designated place (CDP) in Riverside County, California, USA. As of the 2020 census, the CDP had a total population of 3,068, up from 2,534 at the 2010 census.

Winchester was founded in 1886 in Pleasant Valley in what was then San Diego County. The town was named after the widow of Horace Winchester, Mrs. Amy Winchester.

Largely rural for most of its history, and home to agricultural businesses including the Winchester Cheese Company, Winchester experienced rapid growth during the housing construction boom in the early-to-mid 2000s. However, construction and growth slowed when the housing bubble burst in 2007, resulting in a housing market correction. The mid-to-late 2010s saw housing prices in Winchester recover and construction continue with new subdivisions, schools, and parks being built.

In 2022, Riverside County expanded the Winchester Planning area to 42 square miles. extending Winchester to the Temecula City limits. In order to meet state requirements for 40 thousand new homes. The City of Temecula is pushing back against the expanded planning area due to traffic concerns. The city of Menifee is pushing back against the expanded community boundary due to desire to annex land west of State route 79

Winchester is located at 33°42′28″N 117°5′13″W / 33.70778°N 117.08694°W / 33.70778; -117.08694 (33.707871, -117.086861).

According to the United States Census Bureau, the CDP has a total area of 8.0 square miles (21 km²), all of it land.

It is approximately 8 miles southwest of the city of Hemet, California.

Winchester first appeared as a census designated place in the 1990 U.S. census.