Min-Soo Kang opened the scoring in the 18th minute but Ki-Hun Yeom equalised nine minutes before the break.
It was in vain, however, as Sung-Hwan Kim grabbed the winner on the stroke of half-time and Ulsan moved further clear of Pohang Steelers.
In the other game in the Champions Group, Jeonbuk Motors beat Incheon United 2-0 thanks to goals from Jeong Hyuk and Dong-Gook Lee.
In the relegation group Jong-Eun Lim scored the only goal to move Chunnam Dragons eight points clear of Gangwon FC who currently occupy the relegation play-off spot.
Daegu handed their survival hopes a boost as they claimed a gutsy goalless draw away at Seongnam Chunma.