Big Mouth (Lakota: Itȟáŋka) (born 1822–died October 29, 1869) was an Oglala-born chieftain of the Brulé Lakota, highly regarded by the Brulé for his bravery and aggressive military leadership. He was one of the chiefs who signed the second Treaty of Fort Laramie in 1868 and remained a bitter opponent of further American settlement, ridiculing Spotted Tail and other Sioux chieftains upon their return from a mission to Washington, D.C.. He was sixth son of Old Chief Smoke (1774–1864) and twin brother of Blue Horse.
One of the principal chiefs at the Whetstone Indian Agency, located along the Missouri River, where most of the Brulé and Oglala bands had gathered, Big Mouth gained increasing support for his stance among members of the tribe. He criticized what he described as Spotted Tail's reversal of Sioux policy, saying Spotted Tail had been entertained by American politicians and given a personal tour through the major cities of the east coast. Faced with increasing opposition to his leadership, Spotted Tail visited Big Mouth at his lodge, where, upon approaching the entrance, Big Mouth was seized by two warriors and held down while Spotted Tail shot and killed him.