The Black River (French: rivière Black) is a river in the municipality of Sioux Narrows-Nestor Falls, Kenora District in Northwestern Ontario, Canada.

[1] It is part of the Hudson Bay drainage basin, and is a tributary of the Lake of the Woods.

The river begins at an unnamed lake in geographic Work Township[2] and heads southwest through Ulster Lake and under Ontario Highway 71 to Black Lake.

It continues southwest, passes into geographic MacQuarrie Township[3] then Yellow Girl Bay 32B Indian Reserve (part of the Naotkamegwanning First Nation), and reaches its mouth at Long Bay on the northeast side of the Lake of the Woods.

The Lake of the Woods flows via the Winnipeg River and the Nelson River to Hudson Bay.