Everton Santos' first-half goal was enough for Seongnam Ilhwa Chumna to sink Sunderland in their opening pre-season match in the Peace Cup.
Seongnam were given the lead on the half-hour mark when Santos fired home from close range, beating Black Cats keeper Simon Mignolet.
The home side started strongly, not unexpected as they are midway through their season, with Sunderland fading in the humid conditions in the second half. Sunderland's best chance in the first period came in the 16th minute when Kieran Richardson's strike flew narrowly over the bar.
However, Santos and strike partner Javier Reina were a constant threat, squandering several chances before Santos scored what turned out to be the only goal of the game.
In the second half, Seongnam continued to dominate while Sunderland tested new combinations, including the introduction of defender Carlos Cuellar who made his first appearance.
Martin O'Neill's side pushed for an equaliser late on but Connor Wickham's deflected effort hit the post and Craig Gardner came close with a 30-yard free-kick which struck the bar.
The visitors tired in the final few minutes, as the Korean side held on to claim a deserved win.
Matthew Etherington converted from the penalty spot as Stoke began their pre-season schedule with a 1-0 victory in Switzerland against FC Sion.
Etherington slotted home after 38 minutes for the only goal of the game after Jose Goncalves handballed under pressure from Kenwyne Jones.
Potters boss Tony Pulis gave 20 players game time in the match, with only Rory Delap and Jones handed a 90-minute workout.