The Yaak River (spelled Yahk River in Canada) is a tributary of the Kootenai River in Montana.

The Yaak River originates near Yahk Mountain, in the Yahk Range, part of the Purcell Mountains, in southeast British Columbia. The river flows south, crossing into Lincoln County, Montana. It receives the East Fork Yaak River, then the West Fork Yaak River (also called the West Yahk River). The West Fork originates in Montana near Rock Candy Mountain, flows northeast into British Columbia, then southeast back into Montana to join the main Yaak River.

Below the West Fork confluence, the Yaak River receives the South Fork Yaak River before curving broadly west, then south, receiving numerous tributary creeks such as Spread Creek, Hellroaring Creek, and Burnt Creek (also called Burnt Grizzly Bear Creek), before flowing into the Kootenai River near Yaak Mountain and the small city of Troy, Montana.

In Montana, the Yaak River and its tributaries mostly lie within Kootenai National Forest.

The river has Class IV-V whitewater. The river is Class III-V from Yaak Falls to its confluence with the Kootenai River for the purposes of public access for recreational purposes.

According to British Columbia's Geographical Names Information System, "Yahk" is a Kootenai word meaning either "arrow" or "bow" and referring either to the Yaak River or the Kootenai River. The southward curve of the Kootenai River (from Canada into the United States and back into Canada) is said to be a "bow", with the Yaak River possibly being the "arrow" (if the name is from the Kootenai word "a'k").

According to the USGS, variant names of the Yaak River include A'ak, Yaac, Yahk, Yahkh, and Yak.

• Montana Stream Access Law
• List of rivers of Montana
• List of rivers of British Columbia