Mauricio Molina's 16th-minute strike was enough to edge Seoul, who were confirmed as title winners on Wednesday, to three more points today.
The win lifted Seoul 15 points clear of Jeonbuk, with the latter having three games remaining.
Elsewhere in the championship round, third-spotted Suwon Bluewings saw off Busan I'park 2-1, first-half goals from Stevica Ristic and Kim Do Heon proving too much despite Lim Sang Hyub's last-gasp consolation.
That result put Suwon five points ahead of Pohang Steelers, who were held to a 3-3 home draw by Gyeongnam, in the race for the final AFC Champions League berth.
Pohang took the lead three times today - twice through Park Sung Ho and once courtesy of Lee Myung Joo - but were pegged back on all three occasions by goals from Lee Jae An, Jung Dae Sun (penalty) and Kim In Han.
Fifth-placed Ulsan Hyundai twice battled from a goal down to earn a 2-2 draw at Jeju United, Go Seul Ki and Lee Seung Yeoul cancelling out efforts from Song Jin Hyung and Heo Jae Won.
In the relegation round, a late equaliser from Kim You Sung earned Daegu FC a 2-2 home draw against Incheon United. Lee Ji Nam had earlier got the hosts back on level terms after Lee Kyu Ro's opener, only for Ivo Olivio to restore the visitors' lead.
Finally, Daejeon Citizen and Gwangju played out a 1-1 draw. The goals arrived in two second-half minutes, Joao Paulo swiftly equalising for Gwangju after Kim Chang hoon had opened the scoring.