Great Mills High School is a comprehensive public high school in Great Mills, Maryland, United States. It serves students in grades 9–12 in the mixed rural and suburban area at the confluence of the Potomac River, Patuxent River, and Chesapeake Bay. It belongs to the St. Mary's County Public Schools system, and is associated with two other county high schools: Leonardtown High School and Chopticon High School. The school is accredited by the Middle States Colleges and Secondary Schools and the Maryland State Department of Education. The school houses a STEM program.