Sun River is a census-designated place (CDP) in Cascade County, Montana, United States.

The name was from the Indian word Nataeosueti, translated by the English as "Medicine" or "Sun" river.

Artifacts have been found in the area related to native cultures dating back to 2000 B.C.

Largent and Joe Healy also built a toll bridge across the Sun River to profit from the Mullan Road traffic between Fort Benton and the new gold mines to the south.

According to the United States Census Bureau, the CDP has a total area of 1.8 square miles (4.7 km2), all land.

Like most of Montana outside of the highest mountains, Sun River has a cold semi-arid climate (Köppen BSk) characterised by hot summers with chilly nights, and cold winters with very high temperature variability due to warming by chinook winds contrasting with occasional severe chilling by polar air from Canada.

The wettest recorded month has been May 1953 with 6.74 inches or 171.2 millimetres, whilst in September 2012 not even a “trace” was reported.