Poa diaboli is a rare species of grass known by the common name Diablo Canyon bluegrass. It is endemic to San Luis Obispo County, California, where it is known from about five occurrences in the San Luis Mountains near the coast. The type specimen was collected in Montaña de Oro State Park and the grass was described as a new species in 2003. The grass occurs on rugged mountaintops and north-facing slopes in thin soils covering shale rock within a few kilometers of the coastline. Its habitat includes chaparral, oak woodland, coastal sage scrub, and Bishop pine forest. This species is a rhizomatous or stoloniferous perennial grass growing in mounds with stems up to half a meter tall. The inflorescence is a panicle of flowers up to 10 centimeters long with upright or spreading branches. It flowers in March and April, the spikelets green with webby fibers.