Mangaio Stream is a tributary entering the Whanganui River within Whanganui National Park on New Zealand's North Island. It is encountered during the lower section of the classic Whanganui River Journey, typically on the final day of multi-day canoe trips paddling downstream toward Pipiriki. The stream sits in close proximity to Mangaio Rock, where several original iron eyebolts remain embedded in the cliffs — relics of the era when riverboats used them as winching points to navigate the Whanganui gorge.

The stream's most notable feature for paddlers is its narrow ravine, which conceals a rock amphitheatre approximately 100 metres from the river mouth. The entrance from the Whanganui is narrow enough that the amphitheatre is not visible from the main river, making it easy to miss. Paddlers who venture up the stream are rewarded with a dramatic enclosed rock formation. The ravine itself is tight, and the short paddle in is well worth the detour according to those who have made the trip.

Mangaio Stream is not a standalone paddling destination but rather a side excursion accessed while travelling the Whanganui River. It lies downstream of John Coull Hut and Campsite and upstream of the Pururoto Caves and Autapu Rapid, placing it in the deeper gorge section of the river. This stretch of the Whanganui passes through steep, forested terrain accessible only by water, foot, or helicopter, so paddlers should be self-sufficient and prepared for remote conditions.

The Whanganui River Journey season runs from mid-September to mid-April, which reflects the broader paddling window for this section of river. Guided tours operating on the Whanganui regularly point out Mangaio Stream as a landmark and encourage paddlers to explore it briefly before continuing downstream.