A jury found 19-year-old Xzeniyeju Chukwuezi guilty Tuesday in Suffolk Superior Court first-degree murder and unlawful possession of a firearm.
Authorities say Chukwuezi shot eighth-grader Soheil Turner in the back of the head in the Roxbury neighborhood on May 7, 2009.
At his arrest, Chukwuezi was described by investigators as a known member of the Dudley St. posse.
Police say the boy had no gang affiliations.
Police identified Chukwuezi through witnesses and surveillance video. His lawyer says the person in the videos was not his client.
Soheil Turner, a student at Warren-Prescott School in Charlestown, never had any run-ins with police, and was said to be a quiet kid who loved sports.