Harrison has spent the last two seasons on loan at Elland Road, making 91 appearances and scoring 10 goals in all competitions.
The Englishman told Sky Sports News: "Obviously it's nice to be back here at a club where I've been settled, where I've been playing in the last two years and I know everything that goes on here. I'm delighted to be back here again next year, especially in the Premier League.
"I think for myself and the club we achieved something massive. Making it to the Premier League was always our ambition and now we're there we have to set ourselves new goals, myself included, and I'm just excited to get going."