Tottenham Hotspur midfielder
The Premier League club confirmed the switch in a brief statement on its website.
Bentley joined Spurs from Blackburn Rovers in 2008 for a then club record fee of £15 million ($23.25 million) having started his professional career at its north London rival Arsenal.
The 28-year-old has found first-team action at White Hart Lane hard to come by, though, and has spent time out on loan at Birmingham City and West Ham United.
Bentley, who has won seven caps for England, underwent surgery after suffering a serious knee injury while at West Ham last October.
West Ham United has completed its first transfer of the post-season after signing Romania international fullback Razvan Rat on a free transfer.