Onilahy is a river in Atsimo-Andrefana and Anosy (Toliara Province), southern Madagascar.

Two species of cichlids are endemic to the river basin, but Ptychochromis onilahy is probably already extinct[4] and the remaining range of Ptychochromoides betsileanus covers less than 10 square kilometres (3.9 sq mi).

It flows through Ankilimary, to Benenitra, Ehara, Bezaha and Antanimena.

The mouth of the Onilahy river is situated in the Indian Ocean at Saint Augustin, Madagascar, 35 km south of Toliara (Tuléar).

From the north these are Sakondry, Taheza, Sakamare and the Imatoto rivers.