The club announced on Thursday the versatile 28-year-old had left by "mutual consent" and "with the best wishes of all at the club for his future career".
McLean made 34 appearances for the Tayside club after joining from Preston on a two-year contract last summer.
Manager Jackie McNamara, McLean's former Falkirk team-mate, took former Chesterfield central defender Neal Trotman on trial earlier this week.
McNamara had earlier signed centre-back Calum Butcher, while Paul Paton, a summer arrival from Partick Thistle, can play the right-back position that McLean sometimes filled.
United, who are trying to secure a deal for former Portsmouth forward Nadir Ciftci to join the club, dropped their interest in Graham Carey and Jean Yves Koue earlier this week.