A firearm registered to notorious cop killer Christopher Dorner was recovered during the arrest of two men suspected in the armed robbery of a $1 million watch … and the crazy thing is Dorner’s been dead for over a decade.

Prosecutors in Los Angeles say Beverly Hills Police officers discovered a handgun registered to Dorner during a Saturday morning search of the suspects’ Airbnb.

The firearm is a Glock 21 .45 caliber handgun … and prosecutors say it was loaded and tucked inside a pillowcase when cops found it during their search. Authorities also released photos allegedly showing one of the suspects holding the gun.

Dorner, of course, is the notorious former LAPD officer accused of murdering four people — including one police officer — back in 2013 in a case that sparked a 9-day manhunt, which ended with Dorner dying in a remote cabin in Big Bear, CA following an intense gun battle with cops.

Prosecutors say the gun was used last week when two South American nationals robbed a man at gunpoint of a $1 million Patek Philippe watch as he ate dinner with his wife and two daughters on the patio of the Beverly Wilshire Hotel.

Unclear how the men obtained a gun registered to Dorner … but prosecutors say cops tracked down the suspects, arrested them and then found the weapon while executing a search warrant.

Prosecutors say 21-year-old Jamer Mauricio Sepulveda is charged with one count of interference with commerce by robbery (Hobbs Act) and one count of possessing a firearm in furtherance of a crime of violence.

Meanwhile, 19-year-old Jesus Eduardo Padron is charged with one count of conspiracy to commit Hobbs Act robbery.