What country is on the southern border of the USA?

Mexico is on the southern border of the United States. The Mexico–United States border runs from the Pacific Ocean near Tijuana and San Diego to the Gulf of Mexico near the mouth of the Rio Grande. It separates four U.S. states—California, Arizona, New Mexico, and Texas—from six Mexican states.

The border is about 1,954 miles long, although published figures vary slightly depending on how river channels and boundary segments are measured. Much of the eastern section follows the Rio Grande, while the western and central portions cross deserts, mountains, and urban areas. Major paired cities along the border include San Diego–Tijuana, El Paso–Ciudad Juárez, and Laredo–Nuevo Laredo.

The border was shaped by 19th-century agreements following the Mexican–American War and later boundary adjustments, including the Gadsden Purchase of 1854. A common mix-up is to describe Mexico as the only country south of every part of the United States; the U.S. also has maritime neighbors and the southernmost mainland Florida Keys, but Mexico is the country forming its principal southern land border.