Decker is located in Big Horn County 21 miles north of Sheridan Wyoming on the state line along Hwys 338/314 on the banks of the Tongue River.
Located on Montana's southern border, Decker offers access to two state parks. Rosebud Battlefield State Park is the undeveloped site of the 1876 battle between Sioux Indians and General George Crook's infantry. One of U.S. history's largest Indian battles, it set the stage for an Indian victory eight days later at the Battle of the Little Bighorn. Tongue River Reservoir State Park offers boating, camping and fishing.