The Rural Municipality of Coteau No. 255 (2016 population: ) is a rural municipality (RM) in the Canadian province of Saskatchewan within Census Division No. 7 and Division No. 3.
The RM of Coteau No. 255 incorporated as a rural municipality on December 12, 1910.
The RM's eastern boundary runs along the shore of Lake Diefenbaker.
The following urban municipalities are surrounded by the RM.
The following unincorporated communities are within the RM.
In the 2021 Census of Population conducted by Statistics Canada, the RM of Coteau No. 255 had a population of living in of its total private dwellings, a change of from its 2016 population of . With a land area of , it had a population density of in 2021.
In the 2016 Census of Population, the RM of Coteau No. 255 recorded a population of living in of its total private dwellings, a change from its 2011 population of . With a land area of , it had a population density of in 2016.
The RM of Coteau No. 255 is governed by an elected municipal council and an appointed administrator that meets on the second Monday of every month. The reeve of the RM is Terry Sieffert while its administrator is Lindsay Hargrave. The RM's office is located in Birsay.