Joe Malseed saw the 18-inch reptile this week in the city's Fishtown neighborhood and snapped a photo before calling for help.
He says the alligator was in a puddle and appeared to be emerging from a nearby sewer. He says it was docile until a man tried to get a closer look and it hissed.
Police took the reptile to the city's Animal Care & Control Team. The reptile has been named Allie-Gator.
The animal control team said Wednesday despite the name it hasn't determined the alligator's gender because, "Who's going to do that job?"
Allie-Gator will remain with the team until a permanent home is found.