Hammill spent the first half of last season on loan at Huddersfield and has now signed a three-year contract after an undisclosed fee was agreed with Wolves.
The 25-year-old played under Terriers boss Mark Robins at Barnsley and is looking forward to a fresh challenge with the Championship club.
"I'm absolutely delighted that I've been able to come back to the club; for me, it was a 'no-brainer'," Hammill told the club's official website.
"I have a lot of repaying to do for the faith the club and the fans have shown in me; that made a massive impact upon me. The support I got from the club during my loan spell was second to none.
"I'm also delighted to be working with a familiar team and the manager who, in the past, has brought my full potential to the fore.
"The manager knows my strengths and weaknesses; he gave me the freedom to express myself on the pitch and my spell under him at Barnsley was the best of my career to date. I think he deals with players excellently on a one-to-one basis and he's been the right manager for me. I have no doubt that I will thrive under him again."
Robins added: "I know just how much of an impact Adam can have at Championship level and I understand he showed glimpses of that during his loan spell here last season before I arrived.
"Our job - and the challenge for Adam - is to work very hard on the training pitch and get him playing at a consistently high level, because if we can we have a very exciting player on our hands.
"Adam has lots of ability with the ball at his feet and will fit very well into the style of football we want to play, as he can play on either side or with the main striker.
"As everyone knows, increasing our goal-scoring threat is the priority for us at the moment and Adam's arrival is another element of this."