Barnsley boss Paul Hart brought Hassell to Oakwell as part of a rebuilding process in the summer of 2004, completing his move on 1 July. Despite a slow start, Hassell made 43 appearances throughout the 2004-05 campaign and gradually became a fans' favourite. He missed parts of the 2005–06 campaign through injury but played a key role when fit in helping Barnsley gain promotion from League One. Unlike two years' previously, he was on the winning side at the Millennium Stadium as Barnsley triumphed over Swansea City on penalties in the Play-Off Final.