He struck at Westbourne Professional on Edmund Road, Highfield, at 4.15pm yesterday.
Staff at the car showroom said he was dropped off on the back of a motorbike, walked inside and hot wired a Land Rover Defender 110 in the space of six minutes.
The vehicle, with the registration number HT56FHR, is silver and has a contrasting grey roof and grills.
A £1,000 reward is being offered for the safe return of the Defender.
Anyone with information should call South Yorkshire Police on 101 and quote incident 565 of June 27.