The midfielder bagged a late leveller to ensure the Black Cats left the Hawthorns with a point, ruining Alan Irvine's managerial debut for the Baggies on the opening day
A brace from Saido Berahino cancelled out Lee Cattermole's thunderbolt opener for Sunderland and appeared to have handed Irvine a dream start as the new Premier League season got under way.
However, Larsson swept home a low strike in the 85th minute to ensure Gus Poyet's men left The Hawthorns with a point.
Irvine was a surprise choice to replace Pepe Mel as head coach in June and Cattermole ensured a nervy start to the 56-year-old's West Brom reign with a rasping long-range drive after just five minutes.
The 26-year-old collected the ball 25 yards from goal after a Sunderland set-piece and lashed his strike into the top right-hand corner, Ben Foster unable to keep it out despite getting his hands on the ball.
Despite the setback, the home side enjoyed plenty of possession, but failed to create clear-cut chances.
Sunderland should have punished West Brom for their lack of attacking prowess on the half-hour mark when the unmarked Valentin Roberge was picked out by Larsson's teasing free-kick but he could only head wide.
Chris Brunt responded for West Brom with a fizzing strike from distance that went narrowly wide, before new signing Craig Gardner - making his debut against his former club - forced Vito Mannone to tip over the bar with a dipping shot.
West Brom's pressure paid dividends three minutes before the break.
Referee Neil Swarbrick perhaps harshly adjudged that Roberge had tugged down Victor Anichebe in the area and Berahino rolled the resulting spot-kick in the bottom left-hand corner, sending Mannone the wrong way.
West Brom thought they had gone ahead shortly after the restart when Craig Dawson's header appeared to cross the line, despite Fletcher's clearance, but the referee had blown for a foul in the area.
Sunderland were also denied by the officials when Cattermole's cross was flicked by Wes Brown into Fletcher, who headed home at the back post, only to be thwarted by an offside flag.
Berahino then put West Brom ahead.
Graham Dorrans' strike was well saved by Mannone, but Gardner collected the ball on the left of the box and lofted a wonderful cross that Berahino side-footed home at the back post.
Sunderland, however, struck five minutes from time to secure a share of the spoils.
Debutant Van Aanholt found space on the left and picked out Larsson, who applied the low finish from eight yards.