Poecilia gillii, Gill's molly, is a herbivorous livebearer fish distributed throughout Central America.

Poecilia gillii is primarily greyish with blue highlights and rows of yellow spots on the sides.

The individuals living in brackish water tend to be larger than those observed in brooks, reaching up to 105 mm.

[2] In the Pacuare River, a large population was found living over a substrate of algae-covered boulders, pebbles, and sand.

Species sharing this habitat include Amatitlania nigrofasciata, Astyanax aeneus, Parachromis dovii, Poeciliopsis retropinna, Sicydium salvini, and Tomocichla sieboldii.

The copulatory organ, called gonopodium, is longer in the small males in order to facilitate the sneaking up and chasing type of copulation.