The 43-year-old man was attacked as he sat in his VW Golf in Palmerston Road, Buckhurst Hill, Essex on Sunday evening.
The victim, from Tunbridge Wells, was left with serious facial injuries at about 18:45 BST, police said.
His attacker was black, aged 17 to 25, and wearing a dark blue or black hooded top, Essex Police confirmed.
"The robbery happened very quickly, so he was unable to give a detailed description of the suspect," DC Stephen Baker, of Loughton CID, said.
"However, we believe the suspect arrived at the scene in another car and I am appealing for anyone who saw what happened, or has any information to please get in touch."
A thief sprayed a "noxious" liquid in a driver's eyes before forcing him out of his car and driving off.