Formerly, the area of the cities of Rikusentakata and Ōfunato were within the borders of the district. As of 1 June 2019[update] the district consists only of the town of Sumita with a population of 5,228 people, with a density of 15.6 per km2 and an area of 334.84 square kilometres (129.28 sq mi).
November 1, 1926 - The village of Kesen elevated to town status (3 towns, 19 villages)
April 1, 1932 - The village of Ōfunato raised to town status. (4 towns, 18 villages)
April 29, 1940 - The village of Setamai raised to town status. (5 towns, 17 villages)
April 1, 1952 - The towns of Ōfunato and Sakari and the villages of Akasaki, Ikawa, Takkon, Hikoroichi and Massaki merge to form the city of Ōfunato. (3 towns, 12 villages)
June 1, 1952 - The village of Hirota raised to town status. (4 towns, 11 villages)
January 1, 1955 – The towns of Takata, Kesen and Hirota and the villages of Otomo, Takekoma, Yasaki, Yokota, and Yonesaki merged to form the city of Rikuzentakata. (1 town, 6 villages)
April 1, 1955 - The town of Setamai and the villages of Shimoarisu and Kamiarisu merged to form the town of Sumita (1 town, 4 villages)
April 1, 1955 - The village of Tōni annexed by the city of Kamaishi. (1 town, 3 villages)
September 30, 1956 - The villages of Okirai, Yoshihama, and Ryōri merged to form the village of Sanriku. (1 town, 1 village)
April 1, 1967 – The village of Sanriku raised to town status (2 towns)
November 15, 2001: The town of Sanriku is annexed by the city of Ōfunato. (1 town)
