The incident happened at around 4:30 a.m. inside the convenience store at a Chevron gas station, the Yuma County Sheriff’s Office said.
The sheriff’s office released surveillance footage from inside the store that shows three suspects entering the Chevron – with one suspect brandishing a handgun.
The armed suspect walks next to a lone store customer while pointing the weapon toward the cashier, according to the 8-second video clip. The customer acts immediately, lunging at the armed suspect.
Deputies said the customer, whom they identified as a Marine Corps veteran, successfully disarmed the suspect and detained him until law enforcement could arrive.
When responding deputies asked the customer how he was able to take control of the dangerous situation, he stated: "The Marine Corps taught me not to [mess] around."
No injuries were reported and authorities did not immediately release the name of the former Marine.
The juvenile suspect was booked into the Yuma County Juvenile Justice Center on one count each of armed robbery and aggravated assault.
Deputies are seeking the two other suspects, who had fled the scene when the Marine veteran intervened.