The incident happened around 12:00 p.m. Wednesday in the 100 block of Fourth Street in Darby.
Police say the victim, a 17-year-old boy, got into an argument with another 17-year-old boy. Both are Penn Wood High School students.
When officers arrived on the scene they took the victim to the hospital. The suspect fled on foot.
Police were able to find out the suspect's name and are investigating.
The victim was stabbed in the upper left part of his chest and was last reported in stable condition.