Choi Kang-hee's men headed into their game at Jeju on Saturday knowing three points would be enough to seal the crown and they did not fluff their lines, coasting to a comfortable win.
A 28th-minute strike from Brazilian midfielder Leonardo gave the visitors the lead before Lee Seung-ki and Lee Sang-hyup hit home to give Jeonbuk an unassailable 13-point advantage over Suwon Bluewings, who subsequently slumped to a 1-0 home loss to Seoul on Sunday.
Ko Yo-han netted in the fourth minute of injury time to give Seoul a dramatic win and keep the capital club very much in the hunt for the AFC Champions League places.
With three games to play Seoul are four points behind third-placed Pohang Steelers, who were held to a 2-2 draw by Ulsan a day after their title defence was officially ended by Jeonbuk.
At the wrong end of the table, bottom club Sangju Sangmu missed a big chance to boost their survival hopes, drawing 1-1 with Seongnam.
Seongnam took the lead in the 65th minute through forward Kim Dong-hee, with hosts Sangju unable to pull off a turnaround despite Juna Yang's penalty levelling the score eight minutes later.
That result leaves Sangju three points adrift of Seongnam, who fell into the relegation play-off place courtesy of Gyeongnam's 3-1 win versus Jeonnam Dragons, while Busan I'Park are all but safe following a 1-0 success against Incheon United.
