South of Cabo Corrientes (west of Guadalajara), the Pacific lowlands narrow to a thin strip. The main mountain range in the south of Mexico is the sierra Madre del Sur, which stretches across the states of Guerrero and Oaxaca to the low Isthmus of Tehuantepec, the narrowest part of Mexico at just 220km wide. The north side of the isthmus is part of a wide, marshy plain, strewn with meandering rivers, that stretches from Veracruz to the YucatánPeninsula.

In the southernmost state, Chiapas, the Pacific lowlands are backed by the sierra Madre de Chiapas, behind which is the Río Grijalva basin and then the Chiapas highlands. East of these highlands is a tropical rain-forest area stretching into northern Guatemala. The jungle melts into a region of tropical savannah on the flat, low Yucatán Peninsula. The tip of the peninsula is arid, almost desertlike.