The weapon was properly packed in a hard-sided case along with a box that held 35 bullets, the Transportation Security Administration said.
But it wasn't supposed to be carried on, the TSA said.
Had he declared the gun, the passenger would have been able to include the legally owned firearm with his checked baggage, the agency said.
He apparently forgot.
Agents held the man for Port Authority police who took him into custody.
That makes 13 gun arrests at Newark Liberty International so far this year.
Even with air travel crippled by COVID, eleven people were arrested at Newark on gun charges in 2020, as well as in 2019.
The TSA, which didn't identify the man seized Friday, said it "reserves the right to issue a civil penalty to travelers who have guns and gun parts with them at a checkpoint.
"Civil penalties for bringing a handgun into a checkpoint can stretch into thousands of dollars, depending on mitigating circumstances," according to a statement from the agency."
Whether you have a concealed gun carry permit or not, you cannot carry a gun onto a plane, the TSA said.
CLICK HERE: TSA Explains How To Legally Fly With A Firearm
The agency added:
"Passengers are permitted to travel with firearms in checked baggage if they are properly packaged and declared at their airline ticket counter to be transported in the belly of the plane.
"Checked firearms must be unloaded, packed in a hard-sided case, locked, and packed separately from ammunition.
"Firearm possession laws vary by state and locality and travelers should check into firearm laws before they decide to travel with their guns. Travelers should also contact their airline as they may have additional requirements for traveling with firearms and ammunition."
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