An 11th-minute penalty from Eninho got the ball rolling for the visitors, with Lee Dong-gook (55) and Lee Sung-hyun (64) adding further goals in the second half.
Suwon Bluewings are second in the standings following a 3-2 victory over Chunnum Dragons.
Lee Yong-rae (15) and Everton (40) gave Suwon a half-time lead before Kim Shin-young pulled a goal back for Chunnam after 55 minutes.
Stevica Ristic restored the home side's two-goal advantage a minute later, and although Robert Cornthwaite replied once more for Chunnam, they were unable to find an equaliser.
Jeju United consolidated fourth place with a 5-2 rout of Busan I'park.
Jair and Junior Santos scored two goals each for the hosts with Seo Dong-hyun also on the scoresheet. Han Ji-ho and Yoon Dong-min found the target for Busan.
Gangwon FC remain at the foot of the table following a 3-0 defeat at home to Gyeongnam. Kang Seung-jo, Yun Ii-rok and Rocha Caique were the scorers for the visitors.
Goals from Luis Maranhao, Akihiro Ienaga and Choi Jae-soo earned Ulsan Hyundai a 3-1 win over Pohang Steelers, who replied through Noh Byung-joon.
Daejon Citizen shared a 2-2 draw with Daegu FC. The visitors twice took the lead through Yoo Kjoung-youl and Dinelson, but were pegged back by goals from Kim Hyeung-bum and Kevin Oris.
The match between Incheon United and Seongnam Chunma finished goalless.