Authorities identified the suspect as 16-year-old Alex Herrera, who has been charged with first-degree murder and additional offenses, including possession of a weapon by a minor.
The shooting occurred shortly after 8:30 p.m. Wednesday in Haws Alley off George Street.
Police responding to the scene found the victim suffering from a gunshot wound to the head. He was pronounced dead at the hospital.
The Montgomery County District Attorney's Office identified the victim as Javon Oyola, a ninth-grader at Norristown Area High School.
Investigators said two groups of juveniles met three times throughout the evening to fight, and the third confrontation escalated into gunfire, killing the 15-year-old.
Norristown Police Chief Michael Trail said the shooting was not random.
"It was some sort of altercation between the individual and the victim," Trail said. "It's not a random act of violence. This is individuals that were engaged with one another previous to the shooting."
Investigators said video captured the aftermath of the shooting and showed the alleged gunman running from the area with what appeared to be an object tucked into his waistband.
Police said a cellphone left behind at the scene helped connect Herrera to the shooting. Officers later stopped him nearby and took him into custody.
Witnesses described a chaotic and emotional scene as residents and young people reacted to the violence.
"I just seen a lot of commotion, people running, kids crying," said Trina Wright, who watched events unfold from her porch. "It's unfortunate. Why did this baby lose his life?"
Wright said the tragedy has devastated families throughout the community.
"Too many people are hurting over this - mothers, brothers, aunts, sisters," she said. "Too many lives, too many families."
Residents expressed concern about youth violence as summer approaches.
"It's scary because I don't know what the rest of the summer is going to hold for these kids," said Stephanie Bolger of Norristown. "They don't need to be playing with no guns. There's no need fighting with each other."
Norristown Councilman William McCoy said students and staff are struggling to cope with the loss.
"There's a lot of students that are grieving at the moment and staff members grieving," McCoy said. "A lot of kids went home early and a lot of students didn't go to school today."
The investigation remains ongoing.