Kim Hak-bum's men overcame Seoul in the Korean FA Cup final, winning the penalty shoot-out 4-2 after the 0-0 draw.
It was a much-needed boost for Seongnam, who are sitting in the relegation play-off spot in the league.
They are two points from guaranteed safety, and their game in hand against Incheon United gives them an opportunity to climb above Gyeongnam.
In further good signs, Seongnam were 2-0 winners over Incheon United when the teams met in September.
Gyeongnam face lowly Sangju Sangmu on the final matchday on Saturday, making this clash even more important for Seongnam.
Also on Wednesday, fourth-placed Seoul host Pohang Steelers.
The third-placed Steelers currently occupy the third AFC Champions League place, but they are just three points clear of Seoul.
