The agent in question left a gun unattended in the bathroom of the plane during a flight between Tampa and Indiana. The gun was found by a reporter on board who immediately informed the agent, who then went back into the bathroom and retrieved the weapon.
Ed Donovan, a spokesman for the United States Secret Service, told ABC News that officials are "aware of the incident."
"We take the care and custody of our equipment - especially firearms - very seriously," he said. "We will deal with this matter internally and in an appropriate manner."
The Romney campaign referred all questions regarding the incident to the Secret Service.
The gun was left in the bathroom primarily used by media in the back of the plane. Romney and his staff sit in the front several rows of the plane and the candidate uses the forward bathroom.
It was not immediately clear if the gun was loaded or on a safety lock, but the agent was on active duty at the time of the incident.
Following a campaign event in Indiana, the agent in question never re-boarded the flight back to Tampa.