The highly-rated 17-year-old substitute was brought on to bring much-needed life to Blues' attacking play and he did just that, with his 86th-minute swivelled volley seeing off a Portsmouth side who had appeared set for a battling point.
Fellow promising youngster Jordon Mutch had not long gone closest for City with a 25-yard strike which hit the outside of a post, but Blues, who have now found the net 20 times in the final nine minutes of games, produced yet another late show to move within two points of the automatic promotion places.
Pompey, fighting for their financial survival having been issued with a winding-up order by Her Majesty's Revenue and Customs over an unpaid tax bill, showed plenty of spirit, especially in an evenly-balanced first half which saw Joel Ward bring a super save from Blues goalkeeper Boaz Myhill.
Birmingham's last game in hand following their European exertions finally brings them up to speed with the rest of the Championship, this their 41st of the campaign as opposed to Pompey's 30th.
Using his programme notes ahead of the encounter, manager Chris Hughton wrote: "After tonight's game we will see a true reflection of where we stand in terms of the other promotion candidates."
Thanks to Redmond's late goal, they are now hot on the heels of Southampton and West Ham.
City were unchanged from Saturday's stalemate against Saints while Pompey, with their weekend fixture falling foul of the weather, also retained the same XI who clinched only the club's second away win of the season last time out at Peterborough.
It was Hughton's side who started with the greater purpose, Marlon King's pull-back just failing to find a blue shirt before Keith Fahey tested Stephen Henderson from distance.
Marko Futacs wasted a great chance to test Myhill when he blazed over from 12 yards out from Kelvin Etuhu's low cross.
The encounter fell into a lull through the middle part of the half but Ward went the closest to an opener when he produced the first real save of the game in the 40th minute, Myhill getting down well to his right to parry the midfielder's shot from 10 yards.
Portsmouth finished the first half the stronger of the two sides and although City started the second with renewed purpose, they were still struggling to find a way through.
Hughton turned to teenage sensation Redmond on the hour in a bid to do just that and he certainly raised the home support on a bitterly cold night.
City's best two chances fell on the 70-minute mark.
Portsmouth's experienced full-back Tal Ben-Haim played a misplaced back-pass straight into the path of Adam Rooney, who saw his close-range effort smothered by Henderson.
Then from the resulting corner, the ball broke kindly for Curtis Davies but the five-goal centre-back could only flick narrowly wide from six yards.
Midfielder Mutch then struck the woodwork with a low strike from distance and, as Birmingham turned the screw, Redmond clinched it.
Steven Caldwell's diagonal cross was flicked on by Davies and Redmond did the rest to extend Birmingham's unbeaten record on home soil this term and make it 12 matches unbeaten in all competitions.