Tomeurus gracilis is a species of poeciliid fish native to South America, in Brazil, Surinam and Venezuela.

This species grows to a length of 3.3 centimetres (1.3 in) TL.

It is found in muddy creeks or along the sandy-muddy edges of shallow estuaries.

It has been observed remaining still just below the surface but it prefers to hide beneath aquatic plants.

Fertilisation is internal and, uniquely among the Poeciliinae, the females may then lay eggs placing them individually onto aquatic plants.