Bentley, who spent the latter half of last season on loan at Birmingham City, will arrive at Upton Park hoping to propel the Championship club back into the Premier League at the first attempt.
The midfielder cost the Lilywhites an estimated £15 million from Blackburn Rovers in 2008, and for a short time was considered a first-team regular in the England squad.
However, he was unable to maintain his early form and has not featured heavily in manager Harry Redknapp’s plans for some time.
Prior to the season starting, Bentley’s squad number was changed from 5 to 38, further intimating that he was no longer needed at the north London club.
Hammers boss Sam Allardyce, on the other hand, is looking to bolster his new side’s midfield.
Earlier on deadline day England international Scott Parker went in the opposite direction – having been linked with a move away from Upton Park all summer – for an undisclosed fee.
Bentley, 27, was one of numerous fringe Spurs players who have been linked with the White Hart Lane exit door on the final day of the transfer window.
Peter Crouch has been heavily linked with Stoke City and Sunderland, and Sebastien Bassong has been touted for a return to former club Newcastle United.