Dyfed-Powys Police said the man fell into the River Tawe at Ystradgynlais on Sunday morning at around 10am. His body was later found downstream in the Trebanos area.
In a statement released on Twitter, Ystradgynlais Police said: “A male has fallen into the river at Ystradgynlais at approximately 10am this morning and has sadly be found deceased further along the river in the Tebanos area.
“If you witnessed this incident please can you give us a call on 101.”
Most of Wales has been hit hard by Storm Dennis, with the situation said to be “life-threatening” in the south, where the Met Office issued a red warning due to heavy rainfall and flooding risk until 11am.
Homes, roads and railways have been left flooded following torrential downpours and high winds, with recovery efforts underway to help those affected.
Communities in the worst affected areas, including Rhondda Cynon Taff, Merthyr, Abergavenny and Neath Port Talbot, have had to be evacuated.
The Met Office said that winds of more than 80mph have been recorded across parts of the UK, with the highest measuring 91mph in Aberdaron in north Wales on Saturday.
A total of 156.2mm of rain fell at Crai Reservoir in Powys in the 48 hours from Friday to Sunday morning, it added.
Severe flood warning have been issued for the rivers Neath and Taff in south Wales, as well as the River Teme.
Pictures on social media show the Taff bursting its banks and flooding parts of Pontypridd, while rescue workers were using boats to get families to safety after further flooding in nearby Nantgarw.
The Environment Agency’s (EA) flood and coastal risk management executive director John Curtin said on Sunday that a record number of flood warnings and alerts had been issued, with more than 600 in place across England.
Mr Curtin tweeted: “Breaking: we now have the most flood warnings and alerts in force (594) in England than any other day on record.”
The Met Office has issued amber alerts, meaning people must be prepared to change plans and protect themselves and their family from the storm, for parts of Yorkshire, Wales, Devon, East Sussex and Kent.
Rail services were suspended across south Wales after tracks were submerged by rain, while the line between Derby and Long Eaton was also closed.
Flooding saw the M54 westbound closed at Telford, along with the A616 around Stocksbridge in South Yorkshire and the A52 between Stragglethorpe and Gamston in the East Midlands, Highways England said.
Relief from Storm Dennis is still some distance away, the Met Office’s Mr Dewhurst said, adding that “heavy rain and strong winds” would continue across large parts of the UK on Sunday.
“South Wales will see a lot of rain fall before Dennis moves north later to Scotland and Northern Ireland.”
Earlier, the Ministry of Defence deployed British Army personnel to assist people in West Yorkshire areas badly hit by flooding during last weekend’s Storm Ciara.
The British Red Cross said that with water levels expected to peak on Monday or Tuesday it was ready to respond across the country.
Flights in and out of the UK have also been grounded for safety reasons, with British Airways and easyJet both confirming cancellations.
Two bodies were pulled from rough seas on Saturday, before the worst of the storm hit.
One man was found following a huge search operation off Margate after an early-morning distress call, while another was found at Herne Bay.
Additional reporting by PA