ParalympicsGB hopeful Rosie Bancroft, who swims for the City of Manchester team, had hidden the limb in the back seat of her blue Fiat 500 convertible.
The car was stolen from outside her house in Manchester on Saturday night.
The 23-year-old said her vehicle had been specially adapted, including the positioning of the accelerator pedal, so “we’re hoping they didn’t get far”.
Ms Bancroft, originally from Oxford, is studying at the University of Manchester and coaches the swimming club team.
Born without her lower right leg, she needs the prosthetic for running and training in the gym.
It was tailored to her height and weight after “months of painful fittings” and is of no value to anyone else, she said.
“I can’t really walk very far with my other leg and without a car as well it’s going to be hard to get around.”
— Rosie Bancroft (@rosie_bancroft1) February 25, 2018