Ancistrocarphus keilii is a rare species of flowering plant known by the common name Santa Ynez groundstar. It is endemic to Santa Barbara County, California, where it is known only from three populations in the Santa Ynez River drainage. There are perhaps 180 individuals left in existence. The plant was described to science in 2004. Ancistrocarphus keilii grows in sandy soils in chaparral habitat adjacent to oak woodlands. The plant is considered to be vulnerable to extinction because it is rare, its populations are located in habitat that is likely to be disturbed or developed, and because it apparently has no effective method of biological dispersal.