Christopher Battle, 24, has an active arrest warrant for the homicides, police said Friday.
One suspect is already in custody for the shooting. Terrell Frazier, 22, was taken into custody on Wednesday.
But police said this week they are still looking for at least a dozen more shooters after finding 13 separate guns and 140 pieces of ballistic evidence at the scene.
Arrest made in Grays Ferry mass shooting that left 3 people dead; more suspects still being sought
The shooting happened just before 1 a.m. on July 7 in the 1500 block of South Etting Street. Shocking video from the shooting shows multiple people pulling out guns and opening fire.
Police said 13 separate guns were fired in this incident. In the end, three people were dead and nine others were wounded.
Officials said on Thursday that there was no argument or opposing party at the events. They believe there was one individual among the group who fired a gun, possibly into the air, which caused at least a dozen other people with firearms to begin "recklessly and, really, intentionally," firing their guns.
"I don't think I've ever seen contagious gunfire like this," Deputy Commissioner Frank Vanore said.
Video shows multiple people opening fire during Grays Ferry mass shooting
The victims who were killed have been identified as 23-year-old Zahir Wylie, 19-year-old Jason Reese, and 24-year-old Azir Harris.
Among the victims who were injured are a 15-year-old girl and two 17-year-old boys.
The other shooting victims range in age between 18 to 24 years old.
Anyone with information can call or text Philadelphia police at 215-686-TIPS (8477) or submit an anonymous tip online.