The murder happened around 4:30 a.m. in the 400 block of Vollmer Street.
Investigators say the victim, 68-year-old Don Ly, was preparing his cart for the day when someone stabbed him to death.
"It appears that the assailant came upon him without him knowing, stabbed him several times, he went down, then he was stabbed several more times. So, we don't believe he ever got the chance to fight off, or try to fight off, his attacker," said Capt. James Clark of the Philadelphia Police.
Police are looking at surveillance video from neighborhood cameras. There were no apparent signs of a robbery attempt and Ly's valuables were not taken by the knife-wielding attacker.
"He had money on him, he still had his jewelry, still had his wallet, so - right now - it does not appear to be a robbery," said Clark.
Ly came from Vietnam in 1990. His son, a U.S. Army veteran, issued a statement calling his father a "great family man, warm, caring and a hard worker."
For more than a decade, Ly was a fixture in the heart of the University of Pennsylvania campus at 34th and Walnut, selling fruit to students, faculty and other Penn employees.
"Every morning I get my pretzel I always speak to him. Him and his wife, every day. He's a good man, a good man," said Linda Duval from the Van Pelt Library.
Anyone with information is asked to contact Philadelphia Police at 215-686-TIPS (8477), text a tip to PPDTIP (773847) or through PhillyPolice.com.