The police were alerted to a fight in the Skallberget district in Västerås at around 02:30.
On the spot, they met a man who had been injured by a knife. According to P4 Västmanland he must have been stabbed in the arm.
A woman was arrested at the scene and is now being held by prosecutors on suspicion of aggravated assault.
The two involved know each other, according to the police.
The condition of the man’s injuries is unknown.