The Grattan Massacre, also known as the Grattan Fight, was the opening engagement of the First Sioux War, fought between the United States Army and Lakota Sioux warriors on August 19, 1854. It occurred east of Fort Laramie, Nebraska Territory, in present-day Goshen County, Wyoming.

The Sioux Wars were a series of conflicts between the United States and various subgroups of the Sioux people which occurred in the later half of the 19th century. The earliest conflict came in 1854 when a fight broke out at Fort Laramie in Wyoming, when Sioux warriors killed 31 American soldiers in the Grattan Massacre, and the final came in 1890 during the Ghost Dance War.