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. According to the U.S. Census Bureau, the mean center of U.S. population in 2000 fell within Phelps County. Phelps County comprises the Rolla, Missouri Micropolitan Statistical Area.