The Bureau of Land Management aims to gather the estimated 280 horses in the Fifteenmile Herd Management Area, near Worland, returning about 70 to the wild.
Some wild horses living outside the area could swell the numbers mustered to more than 300.
The area covers about 83,000 acres and its environmental management plan recommends levels of 70 to 160 mature horses.
The last state muster in the area was in 2004.
Records show the bureau has removed 906 wild horses from the area in five previous roundups, dating back to 1984.