The trailer, which was carrying the monkeys to a laboratory in Florida, collided with a dump truck on a motorway in Pennsylvania.
No-one was hurt, but four monkeys managed to escape.
Three have since been found, but one monkey remained unaccounted for on Saturday morning.
The long-tailed macaques are in demand for coronavirus vaccine research.
Pennsylvania State Police Trooper Andrea Pelachick confirmed the hunt for the escaped monkeys in Montour County.
