The plain-capped starthroat (Heliomaster constantii) is a species of hummingbird in the "mountain gems", tribe Lampornithini in subfamily Trochilinae.

Both sexes of all subspecies have a long, almost straight, black bill and a white streak behind the eye.

Immatures have a dark sooty brown throat with grayish white margins on the feathers.

H. c. leocadiae's underparts are paler than the nominate's but darker than pinicola's and its gorget is more pink than red.

[5] Subspecies H. c. pinicola is the northernmost; it is found in Mexico from Sonora south to Jalisco.

The nominate H. c. constantii is found from El Salvador through Honduras and Nicaragua into northwestern Costa Rica.

In addition to nectar, it also feeds on small insects captured by hawking from an exposed perch and by gleaning from vegetation.

It makes a shallow cup nest of plant down with lichen on the outside and typically places it near the tip of a branch high in a tree.