Achomawi (also Achumawi, Ajumawi and Ahjumawi), are the northerly nine (out of eleven) tribes of the Pit River tribe of Native Americans who live in what is now northeastern California in the United States. These nine autonomous bands (also called "tribelets") of the Pit River Indians historically spoke various dialects of one common language, and the other two bands spoke dialects of a related language, called Atsugewi. The name "Achomawi" means river people  and the band historically inhabited the Fall River Valley and the Pit River from the south end of Big Valley Mountains, westerly to Pit River Falls.
The other eight bands that share the Achomawi language have a historic homeland storically lived south of the Achomawi language bands in the Hat Creek valley and Dixie Valley.
Achomawi speaking territories reached from Big Bend to Goose Lake. This land was also home to the closely related Atsugewi peoples. Descendants of both cultures later were forcibly relocated onto the Pit River Reservation. Estimates for the pre-contact populations of most native groups in California have varied substantially. Alfred L. Kroeber estimated the combined 1770 population of the Achomawi and Atsugewi as 3,000. A more detailed analysis by Fred B. Kniffen arrived at the same figure. T. R. Garth estimated the Atsugewi population at a maximum of 900.Edward S. Curtis, a photographer and author in the 1920s, gave an estimate of there being 240 Atsugewi and 985 Achomawi in 1910. As of 2000, the Achomawi population is estimated at 1,500.