The on-loan Everton striker made an instant impact as he netted twice in 13 second-half minutes to fire Leicester to a vital three points. Bruma equalised and then put City ahead with superb strikes while Marvin Sordell twice fired Watford into surprise leads.
The win gives Leicester an outside chance of reaching the top six and ends any hope of a play-off spot for Malky Mackay's side.
The Hornets opened the scoring against the run of play after 27 minutes as substitute Sordell drilled the ball into the top corner from 20 yards but Leicester equalised in the 39th minute when Bruma spectacularly fired into the bottom corner from 30 yards.
Watford regained their one-goal advantage after 40 minutes when Sordell, who took advantage of poor City defending, drilled in his second goal of the half from 18 yards. Ben Mee headed against the crossbar before Yakubu took advantage of a horrible error from Scott Loach to level things up at 2-2 after 62 minutes.
The home side went in front for the first time as Bruma smashed a spectacular 35-yard strike into the top corner after 72 minutes before Yakubu added a fourth a minute later, lobbing Loach who had parried Andy King's strike.
Source: PA
Source: PA