Sam Allardyce has lost his first game as Sunderland manager after Costel Pantilimon's blunder led to Saido Berahino netting the winner at The Hawthorns.
After a goalless first half, it was the home side that came out the brighter as they had a penalty claim waved away shortly after the restart, after the ball connected with Younes Kaboul's elbow.
The momentum was still in the Baggies's favour as Chris Brunt sent in a cross from the left-hand side which should have been collected comfortably by Pantilimon in the Sunderland goal but the Romanian goalkeeper fumbled the cross, which left Berahino with an empty net to score.
The Black Cats keeper claimed he was fouled in the build up, despite a lack of contact with the England international.
After the goal, the Baggies continued to pressure Allardyce's side and played on Pantilimon's previous blunder, with the goalkeeper looking shaky after the mistake.
Sunderland had a penalty claim waved away after Fabio Borini was brought down in the box by Stephane Sessegnon but the referee deemed that the former Black Cats attacker got the ball, despite stretching for it after a poor touch.
Tony Pulis's side held on to win however, keeping their ninth clean sheet in 16 home league games.
West Brom go 12th while Sunderland remain winless this season in 19th.
