Two officers were on patrol at about 4:40 a.m. when a 61-year-old homeless man flagged them down on Macdonald Avenue near Harbour Way, French said.
The man was bleeding from the neck and told the officers he had just been assaulted and that his attacker was still in the area.
The officers located the suspect a short distance away and arrested him.
They later learned that the teen had approached the victim and started a casual conversation with him, but then stabbed him in the neck for no apparent reason, French said.
A broken knife handle was found at the scene, and the teenager was found in possession of a matching kitchen knife, French said.
The victim was taken to a local hospital in stable condition.
The suspect was booked into juvenile hall.