A fine headed brace from defender Curtis Davies saw off Watford and propelled Birmingham into the play-off places in the Championship.
Davies now has four headed goals in his last seven games for the Blues, who started slowly but improved.
He opened the scoring when he thumped a Keith Fahey cross into the net, and then doubled the lead from a corner.
Chris Burke smashed in a rebound to wrap up a win which extended Birmingham's recent good run.
Building on their midweek FA Cup success against Premier League opponents Wolves, and last weekend's 6-0 demolition of Millwall in the league, Chris Hughton's Birmingham started this game slowly.
It was visitors Watford who made the early running, with Hornets' top scorer Marvin Sordell played through on goal.
But Blues goalkeeper Boaz Myhill did well, forcing the attacker wide before seeing Sordell's pull-back cleared.
That would prove Watford's best chance in the 90 minutes as slowly Birmingham began to make an impression on the game.
Burke began to make some headway down the wing and the home supporters had something to cheer when a hanging Fahey cross was powered home by athletic centre-back Davies.
Former Aston Villa man Davies then converted a corner on the hour to effectively put the game beyond Watford's reach.
Burke made it 3-0 when he finished coolly from the edge of the area after Hornets' keeper Scott Loach could only parry a Jordon Mutch effort.
Victory makes it eight games unbeaten in all competitions for Birmingham, who find themselves well placed in sixth.
Sean Dyche's Watford side, however, now find themselves in 18th position - just six points clear of the relegation zone.