Emergency services were called to the incident on Wednesday at Pontneathvaughan, Glynneath, and the young man was taken to hospital.
The Air Ambulance, Water Rescue Unit, Dyfed-Powys Police and fire crews attended the scene.
Sadly, the man from Bridgend died at the University Hospital of Wales on Thursday, Wales Online reports.
A spokeswoman from Dyfed-Powys Police said: “Dyfed-Powys Police received a report of a man in difficulty in the water at Pontneathvaughan, Glynneath, on Wednesday, July 24.
“Police attended along with the fire service and paramedics.
“The 20-year-old man from Bridgend was taken to hospital but sadly died on Thursday, July 25.
“There are no suspicious circumstances to his death.
“His next of kin and HM Coroner are aware.”
A spokesman from Welsh Ambulance Service said: “We were called on Wednesday 24 July at approximately 16:40 to reports of a person needing medical assistance near Glynneath.
“We responded with one ambulance, a crew of our Community First Responders, four vehicles of our Hazardous Area Response Team and we were supported by the Wales Air Ambulance and the Fire & Rescue service.
“One patient was transported by road ambulance to University Hospital Of Wales, Cardiff.”
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