The actor purchased the Vauxhall Corsa in 2000 before he was accepted into Britain's National Youth Theatre, and nicknamed his clunky vehicle The Shed.
The car has been sitting in his parents' driveway since Smith found fame, so he has decided to put it up for sale and hand the money over to a good cause.
He says, "It has seen lots of adventures, not to mention a fair few mishaps. I'll be very sad to see it go."
The proceeds will benefit the Starlight Children's Foundation, an organisation that grants wishes for seriously ill children.