The acorn woodpecker was formally described in 1827 by the English naturalist William John Swainson under the binomial name Picus formicivorus from a specimen collected in Mexico. The specific epithet combines the Latin formica meaning "ant" with -vorus meaning "eating". The acorn woodpecker is one of 24 species now placed in the genus Melanerpes that was introduced by Swainson in 1832.