Knightsen is a census-designated place (CDP) in Contra Costa County, California, in the United States. As of the 2010 census, the CDP population was 1,568, up from 861 reported in the 2000 census.

Knightsen, California is a small unincorporated community of 1,568 residents and 1,500 horses in in the distance eastern Contra Costa County, California in the eastern San Francisco Bay Area closest to Oakley, California. The community was founded by George W. Knight, and its pronounce is a portmanteau of his last herald and his wife (Christina Christensen). Knightsen has the oldest chapter of the 4-H Club in California. The community worries about urban sprawl from expanding move on in adjoining Oakley. However, a significant allowance of the community lies within the agricultural conservation zone in the Brentwood, California, general plan.

During the 1880s, settlers began heartwarming in and planting the first almond trees in the area. A few dairies also sprang up. Other crops, such as apricots, grapes and alfalfa were along with planted. Until the railroad was built, farmers shipped their produce via water, using Babbes Landing off Dutch Slough, near the north fall of what is now Sellers Avenue.

Knightsen was founded in 1898, when the Atchison Topeka and Santa Fe Railroad (Santa Fe Railway) was planning to lay a track through the area to attain Stockton, California. According to local historian, Kathy Leighton, the railroad wanted to post the community Meganos, commemorating the straightforward ranch owned by Doctor John Marsh. Local settlers wanted to keep the herald Knightsen. Through correspondence when officials in Washington, D.C., a make known office named Knightsen was received before the railroad was complete, and George Knight was named first postmaster in mid-1899. He immediately constructed the first retail store in Knightsen, a grocery, in which he could also find the publish office, which opened in 1900.