Isumi District is a district located in Chiba, Japan. As of 2010, the district has an estimated population of 18,593 and a density of 120 persons per km2. The total area was 154.76 square kilometres. The district formerly included all of the city of Katsuura, most of the city of Isumi and a portion of the town of Mutsuzawa. It has been reduced in size through mergers and consolidation to two towns.