Phelps County is a county in the central portion of the U.S. state of Missouri. As of the 2020 United States census, its 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. Much of the county is included within the Ozark Highlands American Viticultural Area. Vineyards and wineries were first established in the county by Italian immigrants in Rolla. Since the 1960s, winemakers have revived and created numerous vineyards in Missouri and won national and international tasting awards.

The first Phelps County Court convened on November 25, 1857, in the John Dillon cabin. The historic courthouse was begun in midsummer of 1860, used as a Union hospital during the American Civil War, and served as the courthouse until February 1994, when all county offices were relocated into the new Phelps County Courthouse, which was dedicated on May 22.

Settlers of European descent established farms along the riverbanks in this area in 1818–1819. One year later, Lieutenant James Abert started the first railroad reconnaissance survey in Rolla. Abert was later to become the first professor of civil engineering at the Missouri School of Mines. The founder of Rolla, Edmund Ward Bishop, was originally a railroad construction contractor in New York. He came to this part of the country in 1853, tasked with building the Frisco branch of the Southwest Railroad.

Because of an urgent demand, Phelps County was created by legislative action on November 13, 1857, from portions of Pulaski, Maries and Crawford counties. A special commission was appointed to select a county seat, with instructions to locate the site on the mail line of the railroad as near as possible to the county's center. Bishop then offered a tract of some 50 acres (200,000 m²) for the official town site, and it was accepted. There was disagreement over the site - the "westerners" wanted Rolla, and the "easterners" wanted Dillon, so the General Assembly did not legally declare Rolla to be the official county seat until 1861. The 600-strong group that favored Dillon signed a protest citing the fact that only two of the three commission members had met to consider the possible sites for the county seat. They contested the decision all the way through the Missouri Supreme Court. Before the high court could make a decision, however, the Legislature took action on January 14, 1860, confirming the location of the county seat at Rolla. Smarting under a considerable amount of criticism concerning the matter, all members of the county court resigned during April 1858, but later withdrew their resignations. It was finally settled in favor of Rolla.

Rolla was officially surveyed, laid out, and named in 1858. Bishop wanted to call it Phelps Center, since his house was the center of the county. John Webber preferred the name "Hardscrabble" for obvious reasons. George Coppedge, another original settler, and formerly of North Carolina, favored "Raleigh" after his hometown. The others agreed with Coppedge on the condition that it should not have "that silly spelling, but should be spelled 'Rolla'." The county seat locating commission designated the area now known as Rolla to be the county seat.

The town of Rolla did not exist when the county was officially created (November 1857); only the houses of J. Stever and John Webber were located in the area. Early court business included the location and opening of roads from the county seat to various places within the state, including St. Louis, Springfield, Jefferson City, Lake Spring, and Salem. In this last road order, dated in July 1858, the use of the name Rolla first appears in the court records. The name was used earlier, in May 1858, in a deed of railroad land to the county.