It happened on the 400 block of South 60th Street in Cobbs Creek around 1 a.m. Saturday.
Police said a man and woman were arguing when the suspect approached them.
That's when the suspect and the boyfriend then began to argue.
They said things escalated, and the suspect allegedly pulled out a knife and stabbed the boyfriend in the side.
The suspect then took off.
The boyfriend was taken to Penn Presbyterian Medical Center where he died a short time later.
The girlfriend was taken into custody for questioning.