Race Director Bill Burke says the London man has been hospitalized in critical condition.
Eleven other people also were hospitalized, but didn't appear to have serious injuries.
More than 3,000 people participated in Sunday's race as the heat spiked into the 90s near Central Park. Participants travel a more than 50-kilometer course.
Burke says hospital officials are trying to determine whether the London man had suffered from a heat-related illness. He was not identified by name.
There has only been one death in the race's 10-year history.
In 2008, a 32-year-old man from Argentina died during the Hudson River swim.