A grab from YouTube shows the police officer arresting the woman
More than 800,000 people have viewed the "Philadelphia police brutality" video of a uniformed officer hitting a woman in the face this week.
"Internal affairs is conducting an investigation," Philadelphia Police Department spokesman Lt. Ray Evers said.
He identified the officer on the tape as Lt. Jonathan Josey, a 19-year veteran of the force who has been hailed as a hero in the past.
The incident occurred after a parade on Sunday when officers stopped a motorist at an impromptu neighbourhood celebration.
Some in the crowd threw liquid on the officers, and Josey turned to confront a woman who may have been involved.
He is seen on the tape hitting her in the face, and moments later, the woman's mouth appears to be bloody. She was cited on the scene for disorderly conduct.
The video of the incident was posted on YouTube shortly after it occurred Sunday and it quickly went viral.
Josey has been placed on administrative duty pending the outcome of the investigation.