any of several dialectally related North American Indian peoples speaking a Penutian language and originally living in an area stretching from the San Francisco Bay region southward to Point Sur, Calif. Traditionally, Costanoans lived in a number of independently organized villages; quasi-tribal groupings were later imposed on them by Spanish colonizers on the basis of their proximity to various Franciscan missions (see Mission Indians). Hence, the Costanoans were renamed the Soledad, Monterey, San Juan Bautista, Santa Cruz, Santa Clara (linked with San José), and San Francisco tribes. A group was also named San Pablo, though there was no mission associated with this group.

The Costanoans' chief sources of food were the sea and streams, which provided mussels, salmon, sea mammals, and seaweed. Rabbits, acorns, and seeds were additional foods. Housing consisted of poles covered with tule (reeds) and brush. For clothing, women wore aprons front and back; men usually went naked.

Costanoan culture was radically transformed after the establishment of missions, as church-imposed tribal assignments mixed members of alien Indian groups and brought about the destruction of most of their institutions. Costanoan descendants numbered more than 4,500 in the early 21st century.