Phelps County is a county in the central portion of the U.S. state of Missouri. As of the 2020 United States Census, the population was 44,638. The largest city and county seat is Rolla. The county was organized on November 13, 1857, and was named for U.S. Representative and Governor of Missouri John Smith Phelps.

Rolla is a city in, and the county seat of, Phelps County, Missouri, United States. Its population in the 2020 United States Census was 19,943. It is approximately midway between St. Louis and Springfield along I-44. Its micropolitan statistical area consists of Phelps County, Missouri.