The Black-capped Gnatcatcher, Polioptila nigriceps, is a very small songbird.

Adults are blue-grey on the upperparts with white underparts, with a long slender bill and a long black tail with white outer tailbands on the uppertail. The undertail is extensively white, showing black only along a thin vertical center line and at the very tip. Males show a prominent black cap. This species is very similar to the California Gnatcatcher and the Black-tailed Gnatcatcher.

Their breeding habitat occurs in riparian woodlands of western and northwestern Mexico. It has occasionally nested just across the border in southernmost Arizona of the United States where it is infrequently found in the summer. They build a small cup nest most often in a horizontal limb of a small tree or shrub. Both parents construct the nest and feed the young. The female normally lays 4 eggs.

These birds are primarily non-migratory, remaining in breeding territories year round.

They forage actively in trees or shrubs, mainly eating insects. They may hover over foliage or fly to catch insects in flight.