The Mzimvubu (also spelled Umzimvubu) is a largely undeveloped wild river in the Eastern Cape Province of South Africa. Its name means "place of the hippopotamus" in Xhosa. The river originates in the Drakensberg at an elevation of approximately 2,050 metres near Matatiele and Mount Fletcher, close to the Lesotho border, and flows roughly 250–400 km in a southeastern direction before emptying into the Indian Ocean through a dramatic gorge known as the "Gates of St John" at Port St. Johns. The catchment area covers approximately 19,853 km².

The river has been paddled by relatively few parties and remains one of South Africa's more remote and challenging waterways. The upper section from the source near St. Bernards Peak Hotel features tight technical creeking, long flatwater training stretches above Cedarville, and large boulder-strewn rapids in steep dolerite gorges — described as resembling a small Zambezi at peak summer flows. The gorge section above the Tsita confluence is rated grade 4/5. The lower section from the N2 highway down to Port St. Johns has been documented in Celliers Kruger's book "Run the Rivers of Southern Africa" and is considered suitable for experienced paddlers only. A common put-in for the lower section is near the village of Dikela, though access points along the river generally require careful planning using topographic maps.

The two major tributaries, the Tina and Tsitsa rivers, join the Umzimvubu in the lower reaches, and all three rivers feature deep gorges with significant waterfalls near their respective confluences. The Wits Explorers Society completed a first descent of the full river in 1990 using a ten-man raft. Subsequent expeditions in 2010 explored the deeper gorges near the falls.

The river's character varies considerably along its length, from manageable flatwater to serious whitewater requiring advanced skills. The upper sections are paddled when seasons and water volumes allow, suggesting flow-dependent access. The lower estuary at Port St. Johns is navigable by yacht and smaller craft, with the area noted for unpredictable weather. Given the remote and largely undeveloped nature of the catchment, paddlers should plan for self-sufficient expeditions with limited infrastructure along most of the river's course.