The Rural Municipality of Old Post No. 43 (2021 population: ) is a rural municipality (RM) in the Canadian province of Saskatchewan within Census Division No. 3 and Division No. 2. It is located in the southern portion of the province adjacent to the United States border, neighbouring Valley County and Daniels County in Montana.
The RM of Old Post No. 43 incorporated as a rural municipality on January 1, 1967.
There is one heritage building located within the RM.
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 Old Post No. 43 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 Old Post No. 43 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 Old Post No. 43 is governed by an elected municipal council and an appointed administrator that meets on the second Thursday of every month. The reeve position is vacant while the RM's administrator is Vickie Greffard. The RM's office is located in Wood Mountain.
The RM is a part owner of the Fife Lake Railway.