Jack replaces previous skipper Russell Anderson who retired from football this year following a season-long injury.
Jack signed a year's contract extension in May following intense speculation about his future at the club.
It is not known whether the captaincy was a stipulation in Jack's new contract.
The move will be a blow for Mark Reynolds and Andrew Considine who have both captained in Anderson's absence.
Considine's position in the starting XI is also under threat from new signing Graeme Shinnie, despite the big man just completing a superb season at left-back.