Pohang headed into their game knowing defeat would see Ulsan crowned champions, and set about their task of ensuring that did not happen by taking a 13th-minute lead through Kim Seung-dae.
Dejan Damjanovic got the fourth-placed visitors back on level terms by converting a 23rd-minute penalty, but Noh Byung-joon swiftly restored the hosts' lead before putting the result beyond doubt 15 minutes from time.
Even with the Steelers winning, Ulsan could have clinched the title with a victory of their own, but they could not hold on to the lead they took in the 22nd minute through Rafinha.
Lee Jung-ho equalised midway through the second half and Jose Fagner completed the turnaround for Busan in the final minute.
The results mean leaders Ulsan will go into their final Champions Group game two points clear of Pohang and with all to play for.
In Wednesday's other Champions Group game, Junior Santos' last-gasp strike earned Suwon Bluewings a 1-0 triumph over third-spotted Jeonbuk Motors.
In the Relegation Group, a 1-1 draw at Gyeongnam condemned Daejeon Citizen to the drop.
Han Kyung-In put the visitors ahead 13 minutes from the end, but the lead lasted only five minutes before Kang Jong-kuk restored parity.
The second automatic relegation spot will be filled by either Gangwon FC or Daegu FC, who battled out a 2-2 draw on Wednesday.
Choi Seung-In scored twice in the last 11 minutes to earn Gangwon FC a share of the spoils after goals either side of half-time - from Leandro Lima and Hwang Il-soo - had seen Daegu forge ahead.
The result left Daegu second-bottom and Gangwon one position and two points better off in the relegation play-off place.
Finally, Kim Dong-sub netted the game's only goal nine minutes from time to hand Seongnam Chunma a 1-0 success at Chunnam Dragons.