Castalon is an unincorporated community in southern Brewster County, Texas, United States. It lies along the Texas-Mexico border on the Rio Grande in Big Bend National Park. The community is 35 miles (56 km) southeast of Study Butte, Texas, and 113 miles (182 km) south of the county seat, Alpine. Castalon, named after nearby Castalon Peak, lies directly across the Rio Grande from the Mexican town of Santa Helena. The village originated as a sub-post of the US Border Patrol, which operated from 1910 to 1914 during a border conflict with Mexico. A post office was established in 1922, with W.R. Cartledge listed as the first postmaster. By 1940, Castalon's population was given as 25.
