The Aztec Ruins National Monument preserves Ancestral Puebloan structures in north-western New Mexico, United States, located close to the town of Aztec and northeast of Farmington, near the Animas River. Salmon Ruins and Heritage Park, with more Puebloan structures, lies a short distance to the south, just west of Bloomfield near the San Juan River. The buildings date to the 11th to 13th centuries, and the misnomer attributing them to the Aztec civilization can be traced back to early American settlers in the mid-19th century. The actual construction was by the Ancestral Puebloans.

The site was declared "Aztec Ruin National Monument" on January 24, 1923, and with a boundary change it was renamed "Ruins" on July 2, 1928. As a historical property of the National Park Service the National Monument was administratively listed on the National Register of Historic Places on October 15, 1966. Aztec Ruins was added to the UNESCO list of World Heritage Sites, as part of the Chaco Culture National Historical Park, on December 8, 1987. The site is on the Trails of the Ancients Byway, one of the designated New Mexico Scenic Byways.