The teenage victim was sitting in his car, a silver Peugeot 106, at the McDonalds Restaurant in Lochside at 11.55 pm when he was approached by a man who asked him for a lift.
When he turned him down, the man, who was in his late 20s-early 30s, assaulted the 17-year-old and smashed a window in his car.
The man then walked off towards Cuckoo Bridge.
He is described as being white, about six foot tall, of big build, with short dark hair which was spiked up and he spoke with a local accent. He was wearing a dark body warmer over a black sports type jacket which had green stripes on the sleeves.
The victim received reddening and swelling to the face as a result of the attack.
Constable Luke Leslie at Dumfries said: “This appears to be a totally unprovoked attack on this 17-year-old lad who was sitting in his car with some friends minding his own business.
“Anyone who can help identify the attacker is asked to call us at Dumfries on 101.”