It happened around 9:15 a.m. Wednesday near the intersection of 40th and Market.
When police arrived they found a 26-year-old man with multiple stab wounds to the back.
Witnesses told officers that the victim and another man were fighting just before the stabbing.
The victim was taken to the Hospital of the University of Pennsylvania in stable condition.
The other man, who has not yet been identified, was taken into custody.