The 19-year-old Watson struck from the edge of the penalty area five minutes into injury time after Kieffer Moore gave the ball away, and then slid on his knees with his jersey off in celebration at Wembley Stadium on Saturday.
Sunderland, a club with a distinguished history in English football, could celebrate returning to the top-flight for the first time in eight years.
For Watson, it was the ultimate farewell gift for his club as he'll be joining Brighton next season.
But after ripping off his shirt and sliding toward the celebrating fans on his knees, he told Sky Sports after the game that his "celebration said it all".
Sunderland defender Luke O'Nien, who needed oxygen treatment early and went off after hurting his right shoulder, later joined in the celebrations with his arm in a sling.
Sunderland join Leeds and Burnley in getting promoted from the second tier to the money-spinning EPL.
The play-off final is the most lucrative one-off match in world soccer, with Sunderland in line for an estimated revenue uplift of around Stg 200 million ($A417 million) in match-day, broadcast and commercial revenue.
"Honestly it's incredible, I don't think I can put it into words," Sunderland goalkeeper Anthony Patterson said. "It's not even sunk in what's happening but I'm gonna enjoy every moment now."
Striker Tyrese Campbell gave Sheffield United the lead and his side dominated the first half.
Spanish forward Eliezer Mayenda equalised in the 76th for Sunderland, three minutes after Sunderland coach Regis Le Bris sent on Watson.
It was a heartbreaking defeat for United, who were relegated from the Premier League last season after winning just three games and conceding 104 goals.
