Heskey, 36, had been training with the second tier club and impressed manager Neil Lennon, who signed former Bolton forward Eidur Gudjohnsen on a free earlier this month.
Heskey, who played for Leicester City, Liverpool, Birmingham City, Wigan Athletic and Aston Villa and represented England 62 times, has been without a club since ending a stint with Australian side Newcastle Jets.
He could make his debut against Blackburn Rovers on Friday.
(Reporting by Martyn Herman. Editing by Patrick Johnston)
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