Police say the suspect, wearing dark clothing, entered the business near Shaw and Golden State at around 4 p.m.
He demanded cash from an employee, then robbed a customer before taking off.
"It's a very busy store, it's a very hectic time right now... to me he was kinda brazen to rob it with so many people inside, normally this store is packed...yeah so they are a little shaken up," said Lt. Charlie Chamalbide.
It's not clear how much money was taken. No one was injured.
Anyone with information is asked to contact the Fresno Police Department.