The Marias Massacre is a now little-known massacre that took place in Montana during the late-19th century Indian Wars between the United States government and the American Indians.

Relations between the Blackfoot Confederacy (comprising the Blackfoot, Blood, and Piegan tribes) and whites had been largely hostile for years. Amidst low-level hostilities, sometime in 1869, Owl Child, a young warrior, stole several of horses from Malcolm Clarke, a white trader. Clarke was able to track Owl Child down and beat him in front of his camp. Humilated, Owl Child with a band of rogue Piegans sought revenge and killed Clarke. The killing of Clarke inflamed the public, leading to General Philip Sheridan sending out a band of cavalry led by Major Eugene Baker to track down and punish the offending party.

On January 23, 1870, they received a scouting report that the group of Piegans, led by Mountain Chief, was camped along the Marias River. They attacked the site, but Mountain Chief had received warning and left the area, so Baker's men instead ended up attacking the camp of Chief Heavy Runner, who had enjoyed friendly relations with the white men. As the men of the camp were mostly out hunting, the raid was a massacre of mostly women and children. A hasty count by Baker's men show that 173 dead with 140 women and children that were captured, while only one cavalryman died after falling off his horse and breaking his leg. Mountain Chief's band, in the meantime, escaped to Canada.

Responsibility for the massacre and failure to capture Mountain Chief's men was placed by many on Major Baker's well-known alcoholism, though in the subsequent controversy General Sheridan expressed his confidence in Baker's leadership, and managed to prevent an official investigation into the incident.