The Poronay (Russian: Поронай, Japanese: 幌内川) is the longest river on the island of Sakhalin in Russia.

It flows in a southerly direction through Tym, Smirnykhovsky and Poronaysky Districts.

Nevel in the East Sakhalin Mountains, flows south through the swampy wetlands of the Tym-Poronay Valley, and enters the Sea of Okhotsk in the Gulf of Patience.

The highest level of flow has been observed in early May, and the lowest in the second half of September.

The river usually freezes in the second half of November, while the ice thaws in late April.