The former Elgin City boss had been in charge since October 2017 and guided the Loons to second place in Ladbrokes League 1 last season.
A club statement issued on Sunday evening said: "Forfar Athletic officials are disappointed to announce that earlier today Jim Weir contacted Chairman Ross Graham to advise him that he wished to tender his resignation for personal reasons.
“As was well documented, Jim was involved in a horrific car accident in mid-September and he has found it hard in recent weeks to work on a full recovery from the injuries sustained, alongside his business interests and the pressures and workload of part-time football management.
“He has therefore reluctantly made the decision to take at the very least a few months away from the game to concentrate on his everyday work commitments and his full recovery.
“Despite the recent run of defeats, Jim was aware that he had the full backing of the board at Station Park and the core fan base had also been supportive of Jim’s efforts to turn the tide.
“The Forfar Athletic directors will meet early in the week to formulate their strategy to get a new man at the helm as soon as possible.”
