Isthmus of Tehuantepec, isthmus in southern Mexico, between the Gulf of Campeche on the Gulf of Mexico to the north, and the Gulf of Tehuantepec on the Pacific Ocean to the south. From gulf to gulf the isthmus is 137 miles (220 km) wide at its narrowest part; and it is 120 miles (193 km) from the Gulf of Campeche to the head of Superior Lagoon, an inlet on the Gulf of Tehuantepec. The isthmus is a broad, plateaulike ridge. Its climate is hot, but Pacific winds often bring relief. The northern side is swampy and densely covered with tropical forest; the Pacific slopes on the south are drier.

Although the isthmus has been considered a favourable transportation route since the early 16th century, it was rejected as an interoceanic canal site because construction of the canal would have been too costly. Instead, a railroad extending 192 miles (310 km) from Coatzacoalcos on the Gulf of Campeche to Salina Cruz on the Gulf of Tehuantepec was opened in 1907. The isthmus is connected to Veracruz, Mexico City, and the Guatemalan border by rail. Salina Cruz is a free port with large dry docks. The town of Tehuantepec is the cultural centre of the Tehuana people, whose social organization is matriarchal and whose gold jewelry is especially distinctive.