Ontario police said the object was discovered during the screening process. Fire officials removed it from airport grounds and was expected to dispose of it.
Airport police evacuated Terminal 4 shortly before 6 a.m., according to Transportation Security Administration agent Nico Melendez. The terminal houses American Airlines, Southwest Airlines and US Airways.
The object turned out to be a replica grenade, but authorities still had to respond like it was the real thing.
"When you see these items we just don't know, so we have to act out of security and out of the safety of passengers," Melendez said. "So it is scary at first when you see something like that because you just don't know."
Melendez emphasized that any kind of replicas of guns, grenades or anything similar are not allowed on planes.
Police and fire officials were on scene. Passengers were let back inside the airport around 6:30 a.m. All airport operations have since returned to normal but it did cause an inconvenience - 10 flights were delayed.
The passenger who brought the replica grenade could be fined by the TSA. There was no word on whether that person will face any charges.