Manager Colin Calderwood said: "Alan will bring a lot of qualities to the group and we're delighted to have him with us.
"I was with him at Aston Villa so I know at first hand the kind of player we are attracting to the club and I am sure he will bring his composure and experience to bear.
"He's played most of his career at the highest level and also knows what is required to be part of promotion-winning squads."