The runner was transported to a nearby hospital where he was pronounced dead, fire department spokesperson Frank Dwyer said.
It was unclear what caused the runner to collapse, though a statement from the nonprofit New York Road Runners said it happened after the man finished the race.
“Our thoughts and sympathies are with the runner's family and those close to them,” said spokesperson Trina Singian.
Singian said medical staff had been placed from start to finish throughout the race course. The group also monitored weather conditions up to and during the race with city agency officials and weather experts. Singian said the temperature ranged from the low 60s to high 70s during the race, which began at 7 am.
A heat advisory cautioning that hot temperatures and high humidity could spark heat illnesses was in effect throughout New York City until 8 pm Saturday. Temperatures exceeded 80 degrees Fahrenheit (26.7 degrees Celsius) as of midday in Brooklyn.
Over 22,000 runners were expected to run from Prospect Park to the Coney Island boardwalk in the Saturday half-marathon, which returned to full capacity for the first time in three years.
Dwyer said six other runners were transported to the hospital Saturday morning. Three had serious, non-life threatening injuries, while the remaining three had minor injuries.