City are 16 points clear of United, who can only take a maximum of 15 points from their remaining matches.
United’s shock loss was in stark contrast to their inspiring performance a week ago when they spoiled City’s party after recovering from 2-0 down to beat their rivals 3-2.
United dominated early possession but, despite the efforts of Romelu Lukaku and Alexis Sanchez, the home side couldn’t break the deadlock.
However, with 17 minutes remaining, the visitors sensationally took the lead when Jay Rodriguez scored from a Chris Brunt corner.
United threw everything forward but were unable to breach West Brom’s defence.
Despite the win, West Brom are still nine points from getting out of the relegation zone.
Earliest Premier League champions
Meanwhile, City have set their sights on rewriting the record books after clinching the title.
Doing so with five matches to go makes City the joint earliest champions in the Premier League era – matching the achievement of the 2000-01 Manchester United team.
City already set a new Premier League record earlier this season by winning 18 successive matches.
With 28 league victories so far, City need three more wins to top Chelsea’s record of 30 Premier League wins in a season.