Allagash Lake is in the North Maine Woods on the boundary of Maine range 14 townships 7 and 8 in the United States.

Allagash Stream flows into the northwest corner of the lake from Allagash Pond, Crescent Pond, and Mud Pond in township 9 range 15.

Allagash Stream overflows the northeast corner of the lake and flows 5 miles (8.0 km) through Little Round Pond into Chamberlain Lake.

Chamberlain Lake originally overflowed through Eagle Lake and Churchill Lake to the Allagash River; but was diverted through Telos Cut to the Penobscot River in the 1850s.

[2] Allagash Lake provides good habitat for togue and squaretail.