San Augustine is located at 31°31'45" North, 94°6'20" West (31.529444, -94.105833).
According to the United States Census Bureau, the city has a total area of 4.81 mi2 (12.46 km2). 4.71 mi2 (12.21 km2) of it is land and 0.10 mi2 (0.25 km2) of it is covered by water. The total area is 2.03% water.
San Augustine is located in the Central Standard Time (CST) timezone, GMT-06:00. It observes Daylight Saving Time (DST).