The jury in the High Court at Timaru rejected Smith's defence he killed 16-year-old William in Oamaru on April 1 last year in self-defence.
The verdict came after six hours' deliberation and was delivered in front of about 50 family members and friends from both boys' families.
The court had been told Smith had spent the evening of April 1 drinking and smoking cannabis and rated his level of intoxication at seven out of 10.
He said he had not expected to meet Lewis that evening and had been walking home when he heard William calling his name.
William had accused him of sleeping with his girlfriend and, although Smith denied the accusation, William insisted on starting a fight over the matter.
Smith said William tried to take his hoodie from him and rob him of anything he had in his pockets in lieu of a fight and claimed he had walked away from the confrontation a number of times.
However, William and another youth stopped him from walking home and he could not run away as his shoe laces were not tied up.
Smith said William had pushed him and abused him before trying to forcibly take his hoodie off him.
Finally Smith pulled a knife out of his pocket and swung it around and into William's back three times before running from the scene.
William also ran a few metres but collapsed and died from blood loss a few minutes later.
Evidence from forensic pathologist Dr Martin Sage showed the three stab wounds had been delivered with moderate force and, without prompt medical attention, any one of them would have caused death.
Both lungs had been penetrated, the aorta partially severed and one kidney and a rib cut part way through.
There were no traces of alcohol, drugs or mind altering medications in blood samples taken from William, he said.
Justice John Fogarty remanded Smith in custody for sentencing on July 17.
