The Grattan Massacre, also known as the Grattan Fight, was the opening engagement of the First Sioux War, fought between 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.

A small detachment of soldiers entered a large Sioux encampment to arrest a man accused of taking a migrant's cow, although such matters by treaty were to be handled by the US Indian Agent. After one of the soldiers fatally shot Chief Mat Wayhi (Conquering Bear), the Brul Lakotas returned fire and killed a total of 29 soldiers, Lieutenant John Grattan, and a civilian interpreter. The massacre, as it was called by the American press, is considered an early and significant event in the Plains Indian Wars.

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.