The plain-capped starthroat (Heliomaster constantii) is a species of hummingbird in the "mountain gems", tribe Lampornithini in subfamily Trochilinae. It is found from Mexico to Costa Rica.
The nominate subspecies has metallic bronze green upperparts with a wide white streak on the rump; the crown is less metallic than the rest. Much of the face is dusky, with a wide white "moustache". The chin is sooty to blackish and the gorget ranges from several shades of bright metallic red to purplish red. The underparts are browish gray with a white belly. The undertail coverts are pale gray with wide white tips. The flanks have a large tuft of white feathers. The central pair of tail feathers are bronze green with dusky ends and the rest bronze green with much blackish at the ends and white edges on their inner margins. Immatures have a dark sooty brown throat with grayish white margins on the feathers.
Subspecies H. c. pinicola is similar to the nominate but has paler underparts and a smaller gorget. H. c. leocadiae's underparts are paler than the nominate's but darker than pinicola's and its gorget is more pink than red.