Other gym users used a defibrillatoron hand on the man until the paramedics arrived.
The man collapsed and complained of chest pain and said he could not breathe " at 11:35 PM BST at Kingsway Mall Gym in John Frost Square.
"The gentleman was on the floor because it was hot, a man was fanning him to cool him down because it was they thought he might just have overheated, "a gym member told Hfrance.fr Wales, who said the man initially " looked ok ".
" But then he complained of chest pain and said he couldn't breathe. "
The ambulance was called at 11:41 pm and PureGym, whose 24-hour Newport site was unstaffed between 10:00 p.m. and 6:00 a.m. param the edics arrived on site at 12:24 a.m.
"We are very sorry to hear of the passing of this patient and would like to express our thoughts and more sincere condolences to their loved ones, "said Lee Brooks of the Welsh Ambulance Service.
"We have sought to be candid with the public about the extreme pressures on our ambulance service this week, and as a result some patients have waited a long time for our help.
"We are looking into the circumstances of what happened and will contact the patient 's family directly. "
PureGym has confirmed that a man in their Newport gym had " medical problems which tragically resulted in his death.
The company said that 'a manager was on the scene within 45 minutes and added: "We extend our deep cond olences to his family and friends.
" This gym is occupied from 6:00 am to 10:00 PM but is unmanned outside of these hours when supported by our central support team, including access to a hotspot and CCTV monitoring.
"This is in accordance with the protocols and procedures of expindustry exploitation.
"We appreciate how distressing this incident is for the gym members at this time and have reached out to them for support ".