Lost River is a city in Custer County, Idaho, United States.

There were 30 housing units at an average density of 3.5 per square mile (1.4/km2).

The racial makeup of the city was 95.6% White, 2.9% African American, and 1.5% from two or more races.

13.3% of all households were made up of individuals, and 6.7% had someone living alone who was 65 years of age or older.

12.5% of all households were made up of individuals, and none had someone living alone who was 65 years of age or older.

There were no families and 6.9% of the population living below the poverty line, including no under eighteens and none of those over 64.