Spencer is an incorporated hamlet in Clark County, Idaho, United States. It serves as the base for the state's opal mine and is called the "Opal Capital of America": the mine proper is located 5–6 miles outside of town. The population was 37 at the 2010 census.

Clark County is a rural county in the U.S. state of Idaho; its county seat and largest city is Dubois. As of the 2020 census, the population was 790, making it the least populous county in the state.