Katsuura is a city located in Chiba Prefecture, Japan. As of 30 November 2020, the city had an estimated population of 16,872 in 8566 households and a population density of 180 persons per km2. The total area of the city is 94.20 square kilometres (36.37 sq mi). Katsuura City is famous for Katsuura Fishing Port, which features one of the top three largest morning markets in Japan.

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 (59.75 sq mi). 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. The district is located in the outer zone of Metropolitan Tokyo, roughly 70 kilometres (43 mi) from the center of the capital.