The 23-year-old former Manchester City youngster is reunited with Terriers boss Simon Grayson, who brought him to Leeds ahead of the 2010/11 season.
"Adam is a good footballer," Grayson told Town's official website. "He handles the ball and has a good energy about him - he had a great season last year and scored some important goals.
"He's a player I admired when he was at Carlisle (on loan) and Manchester City and we're delighted to secure his services to this football club. I think it shows our intent in the level of players we want to sign."
"I think we need to score goals from all areas of the pitch, we can't just rely on the strikers to do it. We need the wide players, the centre halves and the midfield players to chip in. Now we have players who are capable of getting into the box and that can contribute in set-piece situations," Grayson added.