The Union County Courthouse is a government building in central Jonesboro, the county seat of Union County, Illinois, United States. Built in 2013, it replaced a brick structure whose core predated the Civil War.

Europeans first reached the vicinity of Union County by means of a French expedition under Jolliet and Marquette in 1673; they are unlikely to have landed within Union County's current boundaries, but they are known to have explored the lower end of the Upper Mississippi River.^[1]: 264 The first settlement, consisting of two families who built cabins near the present site of Jonesboro in 1803, was followed by a second settlement in the American Bottom in 1805, at which time the area was yet part of Johnson County.^[1]: 266 Hills in the area attracted some settlement in 1811 by refugees from the New Madrid Earthquake seeking stable ground,^[1]: 283 but even at the end of the War of 1812, the white man was much less numerous than the red in this part of the Illinois Territory.^[1]: 268 Union County was formed by a law of January 1818, which specified the location at which the county officials should meet before selecting a county seat.^[1]: 285 Just two months later, the officials accepted the donation of land at the present site of Jonesboro, and a rude log courthouse was quickly erected by one Thomas Cox. Although the frontiersmen advocated for such a simple building, holding that justice could be dispensed equally in primitive buildings and grand, a frame courthouse was built in 1820 to replace the original.^[2]

Jonesboro flourished into the 1850s, but the construction of the Illinois Central Railroad posed a new challenge: the company's engineers requested a plat map of the community, but none being furnished, they bypassed the county seat by a mile, and the new community of Anna was established between Jonesboro and the railroad line. Improved transportation caused Anna to prosper,^[2] and it began to challenge Jonesboro for prominence in Union County. After an unsuccessful attempt to provide for a county-seat vote in 1868 via the state legislature, Anna partisans petitioned the county court for redress in 1870, but their petition was rejected, and the challenge gradually subsided.^[1]: 316

By the mid-1830s, the frame courthouse of 1820 had become insufficient, and the county government ordered its removal and the construction of a brick replacement on the same lot;^[2] the project was completed in 1838 at a cost of $5,000. Although the building was quite fine for the period, it was left altogether unmaintained, and the necessity of constructing a fourth courthouse began to appear before the eyes of the county officials.^[1]: 360 The General Assembly passed an act in early 1853 permitting Union County to borrow $5,000 for construction (not many years after a similar act had permitted the county to borrow $1,000 to repair the previous building),^[1]: 317 and the replacement building opened in 1858 at a final cost of $12,000.^[1]: 360 Two stories tall with a taller central section, the building featured a small tower atop the stepped facade. In 1963, the county expanded the building by adding a two-story stone section to each side.^[2] Despite these additions, the county outgrew the building within half a century, forcing some departments to use offices in other buildings, and more than $10 million in bonds was issued in 2012 to enable the construction of a replacement and the demolition of the old building. Its architects were the St. Louis office of HOK.^[3] The replacement opened in August 2013.^[4]

• Union County website
• Photographs of the previous courthouse at various times

37°27′10″N 89°16′18″W / 37.45278°N 89.27167°W / 37.45278; -89.27167