Harlee Dean has signed a three-year deal at Birmingham from Sky Bet Championship rivals Brentford.
Sky Sports News reported on Wednesday that a £2.2m fee had been agreed for the 26-year-old defender, who also has the option for a 12-month extension in his new contract at St Andrew's.
Dean, who twice won Brentford's Player of the Year award, had entered the final year of his contract at Griffin Park and failed to agree new terms with the Bees.
Harry Redknapp had said Birmingham centre-back Ryan Shotton would not be allowed to complete a move to Middlesbrough until he brought a replacement in.
Dean's signing came less than an hour after Shotton put pen to paper on a three-year deal at the Riverside.
Sky sources also understand Birmingham have seen a £3.5m offer for 28-year-old Bristol City centre-back Aden Flint rejected.
Redknapp had grown frustrated at a lack of signings at St Andrew's this summer, but after loan moves for Carl Jenkinson, Cohen Bramall and Sam Gallagher, Dean has become his 10th acquisition of the transfer window.