A retired cop working as a Long Island high school security guard brought his gun to work without permission — and then left it in a bathroom there, officials announced Wednesday.

Someone discovered a holstered pistol on the counter of a restroom near the east gymnasium at Babylon Junior-Senior High School on Tuesday, sparking a response from emergency management, according to the Babylon Union Free School District.

Investigators later realized the weapon belonged to one of the school district’s security guards, a retired Suffolk County police officer.

“At no time was the safety of our students or staff in jeopardy, as the weapon was unloaded and was quickly recovered. It was determined that the item was unattended for a very brief time,” schools superintendent Linda Rozzi said in a letter sent to parents Tuesday.

“The individual in question has been relieved of his duties, effective immediately. We will take this opportunity to review our security and safety protocols and implement any changes necessary to further safeguard our district.”

The school district declined to provide a name of the guard, but said he was licensed to carry the gun but not allowed to bring it on campus. He has been fired.

“He did not have district authorization to carry a weapon,” spokeswoman Deirdre Gilligan told The Post. “It is district policy that nobody is allowed to have a weapon on premises.”

The gun was not loaded, officials said.

The guard was not arrested or charged, but his gun license will be reviewed, according to a spokeswoman for the Suffolk County Police Department.

“The law states that you’re not allowed to bring it on school grounds,” she said. “There will be a review of gun license because of situation.”