The Rural Municipality of Lone Tree No. 18 (2016 population: ) is a rural municipality (RM) in the Canadian province of Saskatchewan within Census Division No. 4 and Division No. 3. Located in the southwest portion of the province, it is southwest of the city of Swift Current. It is adjacent to the United States border, neighbouring Blaine County and Phillips County in Montana.
The RM of Lone Tree No. 18 incorporated as a rural municipality on December 8, 1913. It was named for Lonetree Lake, whose signature tree was chopped down in 1918. This name was once further rearranged to form Treelon, a post office which operated in the municipality until 1945.
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 Lone Tree No. 18 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 Lone Tree No. 18 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 Lone Tree No. 18 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 Roger Goodall while its administrator is Marla Shirley. The RM's office is located in Climax.