The crew was cleaning up after the 10:20 p.m. flight arrived from Boston on Friday and found the stun gun tucked into the back of a seat.
Port Authority spokesman Steve Coleman tells The Star-Ledger of Newark it's not clear who the weapon belonged to or how it got on the plane. Coleman says there's no indication the weapon had been fired.
Port Authority police turned the stun gun over to the Transportation Security Administration. The agency is responsible for screening those who boarded the flight.
Flights used in the Sept. 11 attacks originated from Boston and Newark.