Union County is a county located in the U.S. state of South Carolina. As of the 2020 census, the population was 27,244. Its county seat is Union. The county was created in 1785.

Union County is included in the Spartanburg, SC Metropolitan Statistical Area, which is also part of the Greenville-Spartanburg-Anderson, SC Combined Statistical Area.

The area that includes Union County was once controlled by the Cherokee Indians and they used it as a hunting ground. Up until recent years, one could find numerous arrowheads with little effort throughout the county.

The first European settlers in Union County came from the backcountry of Virginia and Pennsylvania; more than three-fourths were Scots-Irish Presbyterians. It has been suggested that the first group of pioneers arrived as early as 1751. They settled in the northwestern section of the county near a small river that would later be named Fairforest Creek. According to tradition, Mr. McElwaine, a member of the party looked out at the thick woodlands and exclaimed, "What a fair forest!" At the time of their arrival, wild buffalo and horses abounded as well as panthers and cougars, which were called "tigers" or "tygers" by the settlers. This may be where the Tyger River got its name. Another group of Scots-Irish settlers arrived in the late 1750s, all of whom were Presbyterians of Scottish descent, they consisted of five married couples and their children from the village of Ballinamallard, four married couples and their children from the village of Kesh and two married couples with their eight parents and their total of eleven children from the village of Caledon in County Tyrone, Ireland in what has since become Northern Ireland.

The early settlers established Fairforest Presbyterian Church, the first house of worship in Union County. Around 1754, the Brown's Creek area was first settled, about four miles northeast of the present city of Union. A log church or meetinghouse was built and shared among several denominations that could not yet afford their own separate structures. The county and county seat were named for this "Union" church. Quakers arrived in the mid-1750s and settled the southern portion of the county, establishing Cane Creek Church in the Santuc community, and Padgett's Creek Church in the Cross Keys community. The Quakers left in the early 1800s because of their opposition to slavery. Baptists from North Carolina, under the leadership of Rev. Philip Mulkey, reached the Broad River in Fairfield County, SC in 1759. They relocated to Union County in 1762 and in 1771 formally organized into the first Baptist church in the South Carolina upcountry known as Fairforest Baptist Church. Many Baptist churches throughout the upcountry are descended from this original congregation. The congregation later moved to a site on present day SC Hwy 18 between Union and Jonesville where it remains to this day.

During the first part of the American Revolution, the South Carolina backcountry was fairly quiet. In 1776, Fairforest Church became the hub for Union County. Although divided, it was majority Loyalists. The Fairforest Church became the headquarters for the Loyalists. Following the war the church became Padgetts Creek Baptist Church. Following the fall of Charleston in 1780, the British began focusing their attention on the Carolinas. At least five battles were fought in or near Union County, including Musgrove Mill, Fishdam and Blackstock. The county also produced many notable heroes including Lt. Col. James Steen. The war divided the population between Loyalists and Patriots. This resulted in churches splitting up and settlers moving out of the area. Personal property was damaged by both sides.

Following the war, the county seat was established at Unionville (now Union) and a courthouse was constructed. In 1791, the South Carolina Legislature established a district court that included Spartanburg, Union, Chester, and York counties. The area was called the Pinckney District and its headquarters was established at a central location in Union County. Land was cleared and streets were laid out for a new town that would be called Pinckneyville. A courthouse and jail were built for the new judicial district and a college was to be established in the town.

Local tradition states that Pinckneyville was to be home to the United States Military Academy, but lost to West Point by one vote in Congress. Instead, local historians say that Pinckneyville was considered as the site for a federal arsenal. This was likely the source of the legend. In 1799, the General Assembly decided to restructure the state court system. Subsequently, the Pinckney District was abolished; with the loss of the court and associated businesses, Pinckneyville became a ghost town.