Fort Halleck was a military outpost that existed in the 1860s along the Overland Trail and stage route in what was then the Territory of Idaho, now the U.S. state of Wyoming.

The fort was established in 1862 to protect emigrant travelers and stages transporting mail between Kansas and Salt Lake City, Utah, and named for Major General Henry Wager Halleck, commander of the Department of the Missouri and later General-in-chief of the Union armies.

Pressure by white immigrants and shifting buffalo herds forced the Indian tribes to the Laramie Plains where they came into conflict with travelers on the Overland.

At times in 1864 and 1865 ongoing attacks caused the mail to accumulate at stations in Colorado and at Fort Halleck until it could be transported to Green River via government wagons.

The outlaw L. H. Musgrove was charged in 1863 with murder at Fort Halleck and taken to Denver, Colorado, where he was lynched in 1868 by a vigilante committee.