They released surveillance video of the crime, which happened at the location along the 500 block of Spring Garden Street.
Around 9:16 p.m., authorities say the suspect walked in, pulled a semi-automatic handgun from his waistband and pointed it at the 25-year-old female clerk.
The gunman handed her a plastic bag and ordered her to fill it with money.
He then requested money from the safe but, when the clerk told him it was empty, he allegedly ordered her to the back of the store.
The suspect then took off.
The clerk wasn't hurt.
If you have any information about this crime or this suspect, please contact: Central Detective Division at (215) 686-3093.