river in southeastern Mexico and northwestern Guatemala, formed by the junction of the Pasión River, which arises in the Sierra de Santa Cruz (in Guatemala), and the Chixoy River, which descends from the Sierra Madre de Guatemala.

The Usumacinta River flows northwestward, receiving the Lacantún River and forming the border between Mexico and Guatemala at the Chiapas state border. Below the Mayan ruins of Piedras Negras in Guatemala, the river begins its meandering course through the swampy lowlands of the southern shores of the Bay of Campeche. It forms the Chiapas–Tabasco border and continues generally northwestward. The main arm joins the Grijalva River and empties into the Bay of Campeche below Frontera in northern Tabasco state; the central arm, called San Pedro y San Pablo, flows into the bay at the town of San Pedro; and the eastern arm, the Palizada, empties into the Términos Lagoon in Campeche state. The total length of the main channel, including the Chixoy, is approximately 600 miles (1,000 km). Navigable for about 250 miles (400 km) inland, the Usumacinta has had great economic significance as a means of communication for the towns on its banks and for exporting logs, chicle, and other lowland products.