The yellow-bellied chat-tyrant (Silvicultrix diadema) is a species of passerine bird in the family Tyrannidae, the tyrant flycatchers. It is found in Colombia, Ecuador, Peru, and Venezuela.

The yellow-bellied chat-tyrant is 12 to 12.5 cm (4.7 to 4.9 in) long. It appears large-headed and has long rictal bristles. The sexes have the same plumage. Adults of the nominate subspecies S. d. diadema have a dark olive crown, blackish lores, and a yellow forecrown that continues as a supercilium that becomes thinner and paler as it extends far past the eye. Their face is otherwise dark olive. Their upperparts are olivaceous to brownish olive. Their wings and tail are dusky. Their throat and belly are light yellow and their breast a more olive-yellow. Juveniles have an ochraceous tinge on the back of the supercilium, a rufous wash on their back, and an ochraceous vent.

The other subspecies differ from the nominate and each other thus:

S. d. jesupi: no visible difference from nominate in the field

S. d. rubellula: rufescent brown back and wide rufous edges on the flight feathers

S. d. tovarensis: similar to nominate with brighter yellow underparts

S. d. gratiosa: upperwing coverts have dull rufous tips and flight feathers have rufous edges

All subspecies have a dark brown iris, a black bill, and black legs and feet.