According to the United States Census Bureau, the county has a total area of , of which are land and (0.01%) is covered by water. It is the fifth-largest county by area in Texas. The largest part of Guadalupe Mountains National Park lies in the northwestern corner of the county, including McKittrick Canyon and Guadalupe Peak, the highest natural point in Texas at .
According to the United States Census Bureau, the county has a total area of , of which are land and (0.01%) is covered by water. It is the fifth-largest county by area in Texas. The largest part of Guadalupe Mountains National Park lies in the northwestern corner of the county, including McKittrick Canyon and Guadalupe Peak, the highest natural point in Texas at .