K-League leaders the Pohang Steelers fell to their first defeat of the season, going down 2-1 at home to Ulsan.
Pohang trailed on 25 minutes to an own goal from Shin Kwang-Hoon, but responded well when Ko Mu-Yeol equalised three minutes later.
But Kim Yong-Tae ended Pohang's unbeaten run with six minutes of normal time left, and also made it back-to-back wins for Ulsan, who climbed to third on the table.
Jeju United are second on the table, after a Seo Dong-Hyeon brace saw them to a 2-1 win on the road at Suwon Bluewings.
Incheon United went fourth with a 1-0 home triumph against Gangwon, while bottom side Daegu fought hard for their fifth draw of the campaign in a 1-1 stalemate at home to Daejeon Citizen.
Seongnam recorded a 2-0 home win over Gyeongnam, ending a run of three matches without a victory to climb to eighth.