a mountain range in southern Mexico. It stretches 1,000 km along the Pacific Ocean, from the Balsas River valley to the Isthmus of Tehuantepec, and is up to 300 km wide. The highest peak is Mount Teotepec (3,703 m).

The Sierra Madre del Sur is composed mainly of metamorphic rocks with Precambrian cores; in places it is covered by Neogene lavas and, in inland basins, by Mesozoic sedimentary rocks. It is an earthquake region. The Las Truchas iron-ore deposit is located on the northern slopes. Silver, gold, and antimony are also mined. The upper part of the mountains is covered by coniferous and sclerophyll forests, and the lower slopes have thin tropical forests, as well as shrub thickets and summer green forests. Tropical crops are grown on the southern slopes.