It happened around 4 p.m. at a MetroPCS store located on the 7000 block of Elmwood Avenue.
Police say an armed man in his 30s attempted to rob the store when the MetroPCS employee, who has a permit to carry, shot the suspect.
Police say the suspect died at the scene.
"A male entered the store with a firearm and attempted to rob the store to which (the employee) discharged his firearm at the robbery suspect several times, striking him," said Philadelphia Police Captain Scott Drissel.
No other injuries were reported.
Neighbors here say they heard the shots from inside.
Local activist Joar Dahn says it's unfortunate someone has died but also says people have to protect themselves.
He also says he's grown accustomed to shootings here in the neighborhood.
"He's doing his job and standing his ground and whatever he had to do. But this is the normal in Southwest. It's an everyday thing, nothing new for us," said Dahn.
Police confirm this was the third time recently that someone had come into the MetroPCS store with the intent to rob it.