Police responded to the front of the Walnut Park Apartments in the 6200 block of Walnut Street, just before midnight on Wednesday.
"We're getting information they were involved in an early altercation, possibly today, but we're not certain what the altercation was about, but that's what possibly motivated tonight's shooting," said Chief Inspector Scott Small with the Philadelphia Police Department.
When police arrived, they found the security guard with a gunshot wound to the back. The victim was taken to Penn Presbyterian Medical Center in serious but stable condition.
Officers then confronted the accused gunman, who is a 52-year-old resident of the complex.
Police said the man was hiding in the bushes. After failing to listen to repeated police commands, officials said officers deployed a Taser.
They took him and his weapon into custody. He was also taken to Presbyterian Hospital for treatment of minor injuries.
Officials said the preliminary investigation indicates that the victim and suspect had an earlier confrontation at a nearby corner store. During that confrontation, police said the victim was able to disarm the suspect and turned a weapon over to a third party.
When the victim went back to his apartment complex to complete his shift and the suspect later entered the building, according to police.
As the victim was leaving at the end of his shift, police say the gunman approached and shot him in the back.
The security guard was not armed.
The incident remains under investigation.