Thompson's Island is a alluvial island in the upper Allegheny River. It is located in Pleasant Township, Warren County, Pennsylvania, and is part of the Allegheny Islands Wilderness in Allegheny National Forest. The island's forests contain old growth Silver Maple, Sugar Maple, American Sycamore, and Slippery Elm. Thompson's Island is the site of the only American Revolutionary War battle in Northwest Pennsylvania. Colonel Daniel Brodhead defeated the Senecas in 1779.