The yellow-faced grassquit (Tiaris olivaceus) is a passerine bird in the tanager family Thraupidae and is the only member of the genus Tiaris. It is native to Central America, South America, and the Caribbean.

In 1760 the French zoologist Mathurin Jacques Brisson included a description of the yellow-faced grassquit in his Ornithologie based on a specimen collected in Saint-Domingue (now Haiti). He used the French name Le bruant de S. Domingue and the Latin name Emberiza dominicensis. Although Brisson coined Latin names, these do not conform to the binomial system and are not recognised by the International Commission on Zoological Nomenclature. When in 1766 the Swedish naturalist Carl Linnaeus updated his Systema Naturae for the twelfth edition he added 240 species that had been previously described by Brisson. One of these was the yellow-faced grassquit. Linnaeus included a terse description, coined the binomial name Emberiza olivacea and cited Brisson's work. The specific name olivaceus is Neo-Latin for "olive-green". This is now the only species placed in the genus Tiaris that was introduced by the English naturalist William Swainson in 1827. The genus formerly contained additional species. A molecular phylogenetic study published in 2014 found that the genus was non-monophyletic and in the subsequent rearrangement four species were moved to other genera.

The yellow-faced grassquit was traditionally placed with the buntings and New World sparrows in the family Emberizidae, but molecular phylogenetic studies have shown that it is a member of the tanager family Thraupidae and belongs to the subfamily Coerebinae that also contains Darwin's finches.

Five subspecies are recognised:

It is a small bird with a conical bill, sharper than that of the related seedeaters. It is 10–10.7 cm (3.9–4.2 in) long and weighs about 8–10 g (0.28–0.35 oz), depending on subspecies. The adult male has an olive-green back, and its face and breast are black apart from a bright yellow throat, supercilia, and lower eyelid spot. The rest of the underparts are greyish olive. The beak and eyes are dark, while the legs are grey.

The adult female is slightly smaller on average than the male. It is dull olive-green above and paler grey below, and may have some dark breast smudges. The yellow face pattern is much weaker and duller, and may be almost invisible. The lower part of the beak is dark horn-colored (light grey). Young birds are coloured essentially like the adult female, but duller and greyer. Young males begin to acquire full adult plumage in their first year.

The yellow-faced grassquit has a weak buzzing trilled ttttt-tee call. The song is a varying series of high thin rapid trills. Given for a prolonged time, it is melodious, yet subdued, and often only heard from a short distance away.

It breeds from central Mexico to northern Ecuador and north-western Venezuela, and also on the Greater Antilles and nearby islands. It is not a migratory bird but moves about outside the breeding season; vagrants have been recorded in southern Texas and Florida. It has been introduced to Hawaii.