The incident took place outside Clink Bar in Warwick Street at about 2.15am on Sunday.
Two men are understood to have been involved in an altercation in Clink Bar before they were escorted off the premises.
They then became involved in another disagreement on the street during which one of the men used a weapon - believed to be a knife - in a stabbing motion towards the other man.
The attacker made off in an unknown direction.
The victim, a man in his 20s, suffered slight injuries to his hand.
Officers are keen for anyone who saw the assault or who was in the area at the time of the incident to come forward.
Anyone with information is asked to call Warwickshire Police on 101 or Crimestoppers anonymously quoting incident number 288 of January 22.