Old Agency is a census-designated place (CDP) in Sanders County, Montana, United States.

There were 39 housing units at an average density of 66.4 per square mile (25.6/km2).

The racial makeup of the CDP was 15.79% White, 78.95% Native American, and 5.26% from two or more races.

19.4% of all households were made up of individuals, and 3.2% had someone living alone who was 65 years of age or older.

There were 48.4% of families and 53.2% of the population living below the poverty line, including 55.6% of under eighteens and 50.0% of those over 64.