Charles Alexander Eastman (February 19, 1858 – January 8, 1939, born Hakadah and later named Ohíye S'a, sometimes written Ohiyesa) was an American physician, writer, and social reformer. He was among the first Native Americans to be certified in Western medicine and was "one of the most prolific authors and speakers on Sioux ethnohistory and American Indian affairs" in the early 20th century. Eastman was of Santee Dakota, English and French ancestry. After working as a physician on reservations in South Dakota, he became increasingly active in politics and issues on Native American rights. He worked to improve the lives of youths: he founded thirty-two Native American chapters of the YMCA and helped to found the Boy Scouts of America. He was an early Native American historian.