Chad Schieber, 35, of Midland, Mich., collapsed on the city's South Side, according to the medical examiner's office. He was pronounced dead just before 1 p.m. at a Veterans Affairs hospital.
An autopsy is scheduled for Monday morning.
It was the hottest Chicago Marathon, with the previous record of 84 degrees set in 1979. Organizers shut down the course four hours after the start because of 88-degree heat and sweltering humidity.
Runners were diverted to the starting area, where they were provided with medical attention and cooling misters. Shortages of water and energy drinks were reported along the 26.2-mile route.
"We're seeing a lot of our participants slowing," race director Carey Pinkowski said. "It was a contingency plan we had in place and we decided to implement as a precautionary measure."
An unknown number of recreational runners who made it past the halfway-mark were allowed to continue.
Almost 10,000 of the 45,000 registered runners opted to not race in the heat despite more mist stations, cooling buses and water-soaked sponges.
Much of the nation saw unseasonably warm weather Sunday. Nineteen states tied or broke their record high temperatures, according to Accuweather.
The high in Columbus, Ohio was 91 degrees, breaking the previous high of 88. Flint, Michigan reached 87 degrees today, surpassing the old record mark of 84. Green Bay, Wisconsin, also saw its record high rise, from 81 degrees to today's 84.
Kenya's Patrick Ivuti won the marathon in the race's closest finish. Ivuti, competing in only his second major marathon, leaned at the finish line for an unofficial time of 2 hours, 11 minutes, 11 seconds. He edged Jaouad Gharib, from Morocco, who finished 0.05 seconds behind.
"I had never seen a marathon finish up close that was like that," head referee Pat Savage said. "This was really close, but at the same time you could see that one man was ahead of the other."
In the women's race, Ethiopian Berhane Adere rallied to defend her title. She finish in 2:33:49 after passing a surprised Adriana Pirtea, who had a comfortable 30-second lead after 24.8 miles.
Ivuti and Gharib surged ahead of defending champion Robert Cheruiyot and Daniel Njenga at the 22-mile mark to make it a two-man race. Gharib led for much of the final four miles before Ivuti made a push on the final mile.
"One thing I had in my mind was that everybody is going to face the same heat," Ivuti said. "I had no problem with that because everybody was going through the same thing as me."
The duo traded leads on the stretch run down Columbus Avenue before Ivuti's final push at the line. The race was so close that it took organizers several minutes to determine the official winner.
Njenga finished third, and Cheruiyot was fourth.
Cheruiyot was in position to defend his title, but stomach craps forced him to drop back at the 22-mile mark. Cheruiyot, who last year slipped on the finish line and banged his head on the pavement as he raised his hands to celebrate, finished in 2:16:13.
Ivuti was the fifth straight Kenyan to win the title.
Pirtea, of Romania, waved to the crowd as she listlessly approached the finish line. But a final push on the last mile by Adere caught Pirtea on the final stretch.
"At 40 (kilometers), I could see she was going slowly," Adere said. "I knew if I started working from that point on, there was a possibility to catch her."
Pirtea tried to sprint toward the finish line, but finished three seconds behind.
"I had tears," Pirtea said.
American Kate O'Neill finished third, and Liz Yelling was fourth.