Plumas County is a county located in the Sierra Nevada of the U.S. state of California. As of the 2020 census, the population was 19,790. The county seat is Quincy, and the only incorporated city is Portola. The largest community in the county is East Quincy. The county was named for the Spanish Río de las Plumas, which flows through it. The county itself is also the namesake of a native moth species, Hadena plumasata.