The Little Thompson River is a tributary of the Big Thompson River and thence the South Platte River in the U.S. state of Colorado. The river's headwaters lie in the Roosevelt National Forest. It flows east through the foothills of the Rocky Mountains in Larimer County through the town of Berthoud, Colorado, between Longmont to the south and Loveland to the north. The Little Thompson joins the Big Thompson River near that river's confluence with the South Platte River, near the town of Greeley, Colorado.