After the plane landed safely -- and was met by both the FBI and Denver Police Department -- the item was deemed not a threat, the FBI said.
It is unclear who owned the item, but no one is in custody nor is anyone being sought for questioning.
"Out of an abundance of caution, United flight 1243 was diverted to DIA due to a potential security concern," United said in an earlier statement of the flight, which departed from Newark Liberty International Airport outside New York City. "The plane landed safely, and we are currently working on getting customers rebooked."
The Chicago-based airline said there were no injuries as a result of the diversion.
The aircraft was diverted to Denver International Airport around 9:30 p.m., the airport said.
Out of caution, the airport provided buses to transport passengers off the airplane during the investigation to the terminal area.
Passengers were allowed back on the aircraft, the airport said. According to FlightAware.com, the flight is scheduled to continue to San Diego at 11:59 p.m.
ABC News' Jonah Lustig contributed to this report.