The Chisos Mountains, also known as the Chisos, are a mountain range located in the Big Bend area of the Trans-Pecos region of Texas, United States. The mountain system covers 40 square miles (104 square km) and is contained entirely within the boundaries of Big Bend National Park, making it the only mountain range in the United States fully contained within a national park. The Chisos Mountains are the southernmost mountain range in the mainland United States.