The 28-year-old signed for Tottenham Hotspur in a £17m deal five years ago, but he has admitted that he "made the wrong decision".
"I didn't do it the right way - I was a naive young man and made the wrong decision. I thought I was making the right choice but you can't act like that," he told reporters.
"I apologise for the way I went about it because I love being here, I love playing for this club. I thank them for what they did for me while I was here and it's good to be back. Hopefully I can replay them by coming back and performing for them."
Bentley, who has won seven England caps, will spend the rest of the season at his former club.