Kevin Oris struck the decisive goal three minutes from the end that lifted Jeonbuk above Ulsan and behind Pohang on goal difference only at the top of the Champions Group standings.
Pohang opened the door to be caught up after they were held to a goalless home draw by Busan I'Park.
The leaders are now winless in their past five league games and have played a game more than Jeonbuk.
Ulsan and fourth-placed defending champions FC Seoul have both played two games fewer, although the capital club failed to make up ground as they were beaten 2-0 at Suwon Bluewings.
Second-half goals from Junior Santos (56) and Jong Tae-se (81) sealed the points for Suwon.
In the relegation Group, Daejeon Citizen edge closer to the drop as Yoon Suk-young's 45th-minute goal condemned them to a 1-0 defeat at Gyeongman.
Daejeon are nine points adrift of the relegation play-off place after Daegu won 2-1 at Chunnum Dragons.
Hwang Soon-min struck twice in each half either side of a Kim Tae-ho leveller on 52 minutes.
Second-bottom Gangwon were denied a valuable three points by Luis Maranhao's 90th-minute goal at Jeju United. Gangwon had led after Yong Lee put through his own net after 69 minutes.