Police say the stabbing happened on the 1100 block of South Michigan Avenue in the Loop around 7:30 p.m. The location was near the skate park.
The victim, 26, was approached by the suspect, 44, and they started to argue. He then stabbed her in the chest with a "sharp object," police said.
The victim was transported to Northwest Memorial Hospital in critical condition.
The suspect ran off after the incident, but was tracked down shortly after and taken into custody.
Charges were pending, police said.
Police originally said the victim was male. This story has been updated.