Leeds boss Gary McAllister told the club's official website: "We're all delighted. He's a player who's come through the Academy and is a local lad which is a big plus.
"He's got an eye for goal and he has a nice mix to his game. He has a lot of potential and a lot of ammunition to develop his game and I'm pleased I've got the opportunity to help his development."
Howson has scored five goals in 46 appearances for United since making his debut in September 2006.