Achnatherum diegoense is a species of grass known by the common name San Diego needlegrass. It is native to southern California, where it is known from San Diego and Ventura Counties and the Channel Islands, and Baja California. It is a resident of chaparral and coastal sage scrub ecosystems below 350 feet in elevation, especially near streams. This is a bunching perennial grass reaching heights between 110 and 140 centimeters. The flower cluster is up to about 25 centimeters long. The hairy spikelet is about a centimeter long not counting the long awn, which can be up to 5 centimeters long and has two distinct kinks.