|Countries||United States, Canada|
|- left||East Fork Yaak River, South Fork Yaak River|
|- right||West Fork Yaak River|
|- location||Yahk Mountain|
|- elevation||4,910 ft (1,497 m) |
|- location||Troy, Montana|
|- elevation||1,838 ft (560 m) |
|Basin||766 sq mi (1,984 km2) |
|Discharge||for mouth (near Troy, MT)|
|- average||850 cu ft/s (24.07 m3/s) |
|- max||11,600 cu ft/s (328.48 m3/s)|
|- min||49 cu ft/s (1.39 m3/s)|
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 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 tributaries 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.
According to British Columbia's Geographical Names Information System, "Yahk" is a Kootenay word meaning either "arrow" or "bow" and referring either to the Yaak River or the Kootenay River. The southward curve of the Kootenay 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 Kootenay word "a'k").
According to the USGS, variant names of the Yaak River include A'ak, Yaac, Yahk, Yahkh, and Yak.