The Sierra Madre Oriental is a mountain range in northeastern Mexico, spanning 1000 km from Coahuila south through Nuevo León, southwest Tamaulipas, San Luis Potosí, Querétaro, and Hidalgo to northern Puebla, where it joins with the east-west running Eje Volcánico Transversal of central Mexico.
Mexico's Gulf Coastal Plain lies to the east of the range, between it and the Gulf of Mexico. The Mexican Plateau, which averages 1,100 metres in elevation, lies between the Sierra Madre Oriental and the Sierra Madre Occidental further west.
West of the range, the Mexican Plateau is home to deserts and xeric shrublands, including the Chihuahuan Desert to the north, the Meseta Central matorral on the central part of the plateau, and the Central Mexican matorral on the southern plateau.
Although separated from the main part of the Sierra Madre Oriental by the Rio Grande, the Chisos Mountains in Big Bend National Park, southwest Texas are floristically part of the Sierra Madre Oriental, harboring sky islands of pine-oak forest.