The Hanna Formation is a geologic formation in Wyoming that preserves Paleogene fossils, especially in the Hanna Basin and nearby parts of south-central Wyoming. Its fossil record ranges from the Torrejonian to Tiffanian stages of the Paleocene, placing it in the early period after the extinction that ended the age of dinosaurs. The remains found there have helped researchers study Paleocene mammals and date regional rock layers. Because of this, the formation is important both as a local geologic unit and as evidence for early Cenozoic life in the Rocky Mountain region.