The K-League side were given the lead on the half-hour mark when the Brazilian fired home from close range, beating Black Cats keeper Simon Mignolet at the Suwon World Cup Stadium in South Korea.
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 debut appearance for the club.
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.