Police say the shooting stemmed from an argument between two groups having barbecues on Belmont Plateau around 8:00 p.m. Sunday.
According to investigators, a man from one group shot two men in the other group.
One of the victims was pronounced dead a short time after the gunfire.
The other victim was last reported to be in surgery at the Hospital of the University of Pennsylvania.
The suspect fled in an SUV, police said, but was captured a short time later.
His identity has not yet been released.