The man, identified only as Kazimierz, left the marked course during the event, which was held near the town of Marki in record-breaking heat.
He was last seen around the 18km mark.
A large search operation was launched, paused overnight, and resumed on Monday morning.
Unconfirmed reports that a body had been found emerged shortly before 10:00 local time, with organisers confirming the news soon afterwards.
"The search has ended. Unfortunately, it did not bring a happy outcome. A participant in our race, Mr Kazimierz, has died," organisers said on the Poland Bike Marathon's Facebook page.
It is the second fatality linked to Sunday's event.
Another cyclist died after being found far from the designated route, despite paramedics' attempts to resuscitate him.
Race organiser Grzegorz Wajs told public broadcaster TVP3 Warszawa that the rider had strayed far from the course.
"As soon as we found him, paramedics arrived and tried to resuscitate him, but unfortunately they were unable to save him," he said.
Wajs added that the race had never experienced such tragic incidents before.
"There have been cases of riders feeling unwell, but cyclists are used to racing in different conditions and temperatures."
"This was not the first time they had ridden in such heat. We had adequate hydration and water-misting stations in place," he said.
(ał)
Source: tvp.info