Corrie McKeague, 23, was last seen in the early hours of 24 September in Bury St Edmunds, Suffolk. He was later reported missing when he failed to turn up at his base, RAF Honington, ten miles from the town.
His grandparents Mary and Oliver McKeague, from Cupar in Fife, announced the reward as they said they believed his disappearance may involve a third party that he may have gone with.
His grandfather Oliver said: “Corrie’s base at RAF Honington is near two other military bases, RAF Marham to the north, and Aldershot Camp to the south, and each recently reported suspected kidnapping attempts of military servicemen.
“You could draw a near straight line on a map and connect all three. Corrie’s base is right in the middle.”
Grandmother Mary McKeague said: “We’ve pledged this reward in hopes that it will motivate someone to come forward and help us find my beautiful grandson, Corrie, who went missing the day of his stepmother’s birthday.
“We’re heartbroken, but we’re not giving up.
“Corrie would expect us to stay strong.”
The gunner and team medic was separated from friends while leaving the Flex nightclub on St Andrews Street South, his grandparents said.
He was last seen in Bury St Edmunds town centre on CCTV at 3:25am wearing a light pink Ralph Lauren shirt, white jeans and brown suede Timberland boots with light soles.
The last sighting shows him walking from a shop doorway and into a horseshoe-shaped area in Brentgovel Street, with no sign of him emerging.
“The CCTV cameras show him walking in but not coming out,” said Oliver McKeague. “He seems to have just vanished. But people don’t just vanish; they leave or they get taken away.
“We think Corrie arranged to meet someone that night but we don’t know who, and then got into a car and disappeared.
“We know he didn’t just run away; he loved his fellow squaddies, his life and the career direction he had chosen.
“He was so proud the day he passed out and joined 2 Squadron RAF Regiment three years ago, and so were we.”
The McKeague family are asking for information from anyone who may have been in the town on evening when he went missing or who may know of his whereabouts.
This can be given via the Suffolk Police incident room on 01473 782019 or Crimestoppers on 0800 555 111.