It happened at the KARCO gas station at Broad and Clearfield streets around 5 p.m. Monday.
Police say a 39-year-old man approached the security guard with a large concrete rock about the size of a softball.
"They were involved in some sort of verbal altercation, and that's when the 39-year-old male - who was holding the rock - threw it at the security guard," said Chief Inspector Scott Small.
The guard then fired one shot, Small said, but no one was hit. The guard was not hit by the rock.
Police arrested the rock thrower for simple assault and reckless endangerment.
They add both that man and the security guard are cooperating.
Action News reported back in December that the owner of KARCO gas stations had hired armed guards to protect his business.
Philadelphia gas station owner hires armed agents to patrol property
RELATED: Check the 6abc Neighborhood Safety Tracker