State Highway 80 or SH 80 is a state highway in the U.S. state of Texas that runs 83 miles (134 km) from San Marcos to Karnes City. The original highway was designated in 1926 from San Marcos to Luling, with a planned extension south to Nixon. By 1928, SH 80 extended to Wimberley. This absorbed all or portions of previously designated SH 29A and SH 21. By 1940, the route to Nixon had been completed, with the highway extended further south to Karnes City over former SH 112. In 1942, the section from Wimberley to San Marcos was transferred to Ranch to Market Road 12.