Gardner, 26, joins up with brother Craig at St Andrew's, for what will be his seventh loan away from Aston Villa. The midfielder spent last season on loan at Barnsley, and has previously played for Nottingham Forest, Brighton, Sheffield Wednesday and Coventry City, all on a temporary basis.
Since joining the 'Villains' in 2011, Gardner has only made 46 appearances for the club and still has two years left on his contract at Villa Park.
The 'Blues', since having their transfer embargo lifted by the EFL, have been allowed to sign five new players before the deadline.
Steve Bruce's side have already brought in Danish left-back Kristian Pedersen, alongside Lee Camp on a free transfer and Cardiff's Omar Bogle and Bournemouth's Connor Mahoney on loan deals.