Mitsuishi (三石郡, Mitsuishi-gun) was a district located in Hidaka Subprefecture, Hokkaido, Japan.
As of 2004, the district had an estimated population of 4,950 and a density of 14.30 persons per km². The total area was 346.23 km².
On March 31, 2006, Mitsuishi District merged with Shizunai District to create the new Hidaka District. Mitsuishi District and Shizunai District were both dissolved with this merger.