Rio San Antonio is a tributary of the Conejos River in northern New Mexico and southern Colorado.

The river begins at the western boundary of the Carson National Forest, in the Tusas Mountains, in Rio Arriba County, New Mexico.

[4] Before reaching the mountain it turns north, entering the rocky Taos Plateau volcanic field on the fringe of the San Luis Valley.

[5] It enters Conejos County, Colorado at Ortiz, where the Rio de los Pinos joins from the west.

This wilderness area primarily protects a 100-foot gorge along the Rio San Antonio and the surrounding plains at an elevation of 8,400 feet (2560 m).