Old Union is an unincorporated community in Bowie County, in the U.S. state of Texas. According to the Handbook of Texas, the community had a population of 238 in 2000. It is located within the Texarkana metropolitan area.
Old Union is located on U.S. Route 67, southwest of New Boston and southwest of Texarkana in southern Bowie County.
A log schoolhouse was built around 1830. Today, Old Union is served by the Simms Independent School District.