Plumas County is a county located in the Sierra Nevada of the U.S. state of California. As of the 2010 census, the population was 20,007. The county seat is Quincy, and the only incorporated city is Portola. The largest community in the county is East Quincy. The county gets its name from the Spanish words for the Feather River (Río de las Plumas), which flows through the county.