The gun was tucked into the back of a seat on the plane, The Newark Star-Ledger reported Sunday.
The weapon was found Friday as a crew cleaned the plane after it landed in Newark.
Port Authority spokesman Steve Coleman said it was't known who the stun gun belonged to or how it got on the airplane.
The gun was turned over to officials from the Transportation Safety Authority, the agency that would have been responsible for screening passengers before they boarded the aircraft.
There was no indication the electroshock weapon had been fired, officials said.