A relaxed Korir surged past Ethiopia's Milkesha Mengesha at the 20-mile mark to take control of the race.
He then ran alone for the final six miles, breaking the record set in 2011 by compatriot Geoffrey Mutai in 2:03:02. He clocked his last mile of the race in four minutes and 26 seconds.
Before crossing the finish line, a broad smile crossed Korir's face as he stuck out his tongue in a playful gesture.
"I knew I would defend my title, but I didn't know I'd run that fast," Korir told a press conference.
It was a Kenyan sweep at the world's oldest marathon as defending champion Sharon Lokedi won the women's race in two hours, 18 minutes and 51 seconds. She set the course record last year in 2:17:22.
With about five miles to go, Lokedi had Loice Chemnung - also of Kenya - clinging to her pace, but then she dropped her rival with a blistering surge, running mile splits under four minutes and 50 seconds in the last stage of the race.
"I was just like, 'let me push it and see how it goes,'" Lokedi said. "I left it all out there. That's all I can say."
Kenyan women took the top four spots. Jess McClain placed fifth in 2:20:49, the fastest time ever for an American woman at Boston.
It was a chilly start to the 130th running of the race, with temperatures at 45 degrees Fahrenheit (7 Celsius) on a clear day that offered a slight tailwind to the runners.