The Rural Municipality of Grassy Creek No. 78 (2016 population: ) is a rural municipality (RM) in the Canadian province of Saskatchewan within Division No. 3.
The RM of Grassy Creek No. 78 incorporated as a rural municipality on January 1, 1913.
In the 2021 Census of Population conducted by Statistics Canada, the RM of Grassy Creek No. 78 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 Grassy Creek No. 78 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 Grassy Creek No. 78 is governed by an elected municipal council and an appointed administrator that meets on the third Wednesday of every month. The reeve of the RM is Michael Sutter while its administrator is Kathy Collins. The RM's office is located in Shaunavon. The RM shares this office with the RM of Wise Creek No. 77.