The 29-year-old won his fourth marathon in a time of two hours, five minutes and 50 seconds and was wrapped up and getting warm by the time countryman Chebon Chebor crossed the line in 2:10:24.
Benjamin Kolum took third place with a time of 2:10:35.
Kwambai, who picked up $50,000 for the victory, finished runner-up in the Seoul International Marathon earlier this year behind another Kenya, Wilson Loyanae.
Kim Young-in was host South Korea's highest-placed finisher on Sunday, trailing home ninth in 2:17:00.
- Sports & Recreation
- Athletics, Track & Field