The Monument Square Historic District is a historic district on Main and Water Streets, and Grove Avenue in Leominster, Massachusetts. The district includes Leominster's traditional town common or square, which is known as "Leomister Square" or "the Common." The historic district also contains many 19th-century buildings, and the area was added to the National Register of Historic Places in 1982. The city hall and various churches and businesses are adjacent to the square.