Phuket City Police were called to the scene, at the entrance of Soi Vanich 1 in Moo 7, Ratsada, at 10:48pm.
At the location, near a small grocery store, officers found a large pool of blood and a black Honda Scoopy motorcycle believed to belong to the victim.
The young man had sustained deep cuts to his arm and shoulder, though he remained conscious.
Rescue workers from the Phuket Kusoldharm Foundation provided immediate first aid before taking him to Phuket Provincial Hospital for further treatment.
Witnesses told police that a group of seven to eight teenagers on multiple motorcycles pursued the young man and a teenage girl who was with him.
According to onlookers, the teenagers rode in a reckless, aggressive manner, cornering the pair before the man attempted to take cover in the alley.
However, before he could escape, the group confronted him, slashing him with a knife before fleeing the scene.
Police suspect the assault may be related to a prior conflict between the individuals involved, but have yet to reveal details.
Phuket City Police are now searching for the attackers with the intent of pressing charges.