The Welsh international moved to the club in 2011, making 28 appearances during his time in Scotland. However, he will now pursue a career in England.
"Jason made it clear to us he was keen to return to England and continue his career. He is a proud member of the Welsh international team and in order to maintain his place in the squad he feels it is vital for him to be playing every week. The club wish Jason all the best with his future career," he told reporters.
Brown, 30, has three international caps to his name.