It happened some time on Friday night or Saturday morning in Clones.
Police said the blade is a custom fit for the owner and would be of no use to anybody else.
The owner is taking part in an international athletics meeting in Berlin in 10 days and will be unable to take part if it is not recovered.
The blade is for a left lower limb.
A carbon fibre blade -  a running leg for amputees -  has been stolen from a vehicle in County Monaghan.