ACL win puts Wanderers in the box seat
Both goals came in the second half with Mark Bridge scoring the opener with a fine header before setting up another for Brendon Santalab for the sealer 10 minutes from time.
The victory puts the Wanderers on the verge of qualification ahead of the final group game at home to Guizhou Renhe.
Without regular goalkeeper Ante Covic, Jarrad Tyson pulled on the gloves and was quickly off his line to deny Shin-Wook Kim 11 minutes into the contest.
Chances were few and far between in the opening half with Tyson showing safe hands with back-to-back saves from Sang-Woon Han.
With the Wanderers producing little going forward, they had Tyson to thank at half-time after the goalkeeper produced a smart save to deny Brazilian Rafinha from close range.
Two minutes after the interval, Bridge almost capitalised on a poor back pass to give the visitors the lead before finding the back of the net in the 61st minute.
The veteran played the ball out to the left to Shannon Cole, who curled in a pinpoint cross for Bridge to expertly head home from just next to the penalty spot.
The Wanderers were content to hold onto their lead, but Ulsan failed to test the resolute back four with Michael Beauchamp and Nikolai Topor-Stanley very impressive.
The points were sealed in the 80th minute when substitute Brendon Santalab turned in Bridge's cross from close range.
Ulsan Hyundai 0
Western Sydney Wanderers 2 (Bridge 61, Santalab 80)