The former Blackburn and Arsenal midfielder joined Spurs in July 2008 for a fee of £15million, but has struggled to make an impact at White Hart Lane.
The 26-year-old underwent a medical at St Andrew's earlier this week and has now completed a deal to join Alex McLeish's side.
Subject to his registration being accepted by the Premier League and Football Association, Bentley will be available for Sunday's derby at home to Aston Villa.
Source: PA
Source: PA