Jeju came from a goal down to shock Japan's Gamba Osaka in Group E of the AFC Champions League.
Sota Nakazawa gave the visitors the lead on 23 minutes, but Jeju hit back hard after the break, with second-half goals to Shin Young-Rok and Bae Ki-Jon helping the Koreans move into second place in the group with six points.
Cerezo Osaka moved level with AFC Champions League Group G leaders Jeonbuk Motors on Tuesday following a 1-0 win against the Koreans.
Takashi Inui proved the difference at the Kincho Stadium in Osaka, scoring the only goal of the game eight minutes after half-time to delight the home supporters and lift his side to two wins from their opening three matches, level with Jeonbuk on six points.
Group G's other match saw Indonesian outfit Arema play out a 1-1 draw against Chinese Super League champions Shandong Luneng in Kepanjen.
Former Flamengo striker Obina silenced the home supporters when he fired Shandong ahead after only eight minutes, but the hosts responded in second-half injury time through a penalty from Moch Fakhrudin to ensure honours remained even.
Shandong slipped to third place in Group G with the draw, while Arema remain anchored to the foot of the table after registering their first point of the campaign.
In Group E, Chinese side Tianjin Teda moved a point clear on top thanks to a 1-1 draw with Australian side Melbourne Victory.
A header from Kevin Muscat cancelled out Marko Zoric's opener for Tianjin to hand Victory their first point of the campaign, while the Chinese team moved a point clear of Koreans Jeju United at the top.
The Australians will be ruing a missed opportunity after dominating the second half in Tianjin, particularly after the hosts were reduced to 10 men following Zoric's dismissal for a crude challenge on Carlos Hernandez.
Gamba are in third place with three points, and Victory are bottom of the standings with a solitary point.
In Group C, Uzbekistan's Bunyodkor played out a 0-0 draw against Iranian side Persepolis in Tashkent.