The 26-year-old has joined the Club from Aston Villa for an undisclosed fee.
Gardner has penned a three-year contract.
The midfielder has just come off the back of the most productive season of his career, on loan to Blues from Villa.
And the Solihull-born Blues fan was always of a mind to make the switch for keeps, if possible.
Gardner made 39 Championship starts and one substitute appearances this season, netting twice.
He fitted in seamlessly to Garry Monk’s side and produced consistence performances in the heart of midfield.
Gardner also created a bit of Blues history when he and brother Craig both played in the same side, becoming only the second pair of siblings to do so for the Club.
Gardner began his career at Villa, coming through the Academy ranks to make his senior debut in December 2011, after earlier spending time at Coventry City on the first of many loan spells.
Gardner has also been loaned out to Sheffield Wednesday, Brighton & Hove Albion, Nottingham Forest and Barnsley, before finding his true calling at the St. Andrew’s Trillion Trophy Stadium.
He became a Villa regular in the 2016/17 season after their Premier League relegation, only to again find himself out of favour thereafter.
Gardner battled back from two serious knee injuries during his career and his conditioning and availability was never in question during this campaign.
He missed the derby games due to the terms of his loan and appeared in both Blues cup fixtures.