The Capitan Formation is a geologic formation found in western Texas and southeastern New Mexico. It is a fossilized reef dating to the Guadalupian Age of the Permian period. The formation underlies El Capitan in Guadalupe Mountains National Park, and the formation and its associated basin, shelf margin, and backreef formations have been described as "the largest, best-preserved, most accessible, and most intensively studied Paleozoic reef complex in the world."