Archie Wood, 12, was metal detecting on a Bexhill beach with his father when he spotted a strange item sticking out of the sand.
His grandad, Neil Wood, said: “He was so excited, he couldn’t believe what he was digging up.
“He saw it sticking out the sand. When he started digging he pulled out this enormous tusk.”
At first they thought it might be a large piece of wood, but it was too heavy and looked as though it was once part of a jaw.
Archie, who lives in Hampden Park, Eastbourne, got a metal detector as a present for Christmas and he has always been fascinated by fossil hunting and making discoveries.
Mr Wood continued: “He was ecstatic. He couldn’t rush home quick enough to show it.
“Everybody that’s seen it has gone ‘wow!’ If you feel it it’s amazing, it’s very heavy.
“You can see it was once part of a jaw. It could have belonged to a sabre toothed tiger or a mammoth.”
They have sent pictures of the find to local experts and to the Zoological Society of London, and are waiting to find what they make of it.