Daniel Hume (15) of Bures Road, Great Cornard, found what is thought to be one of only a very small number of Valdosaurus thigh bones which have been discovered.
He was taking part in an organised fossil hunt with his mother Debby and brothers Ben (14) and Sam (12) during their holiday at Brook, on the Isle of Wight.
While looking in a pool on the beach he noticed the black, figure-of-eight piece of bone and took it to the dig leader.
Daniel said: "It was exciting. I am interested in stones and fossils so I was hoping to find something on my trip, but I didn't expect to find anything rare.
"I could see that what I had found had all the tell-tale signs of being a piece of fossil, such as honeycomb patterns where blood vessels would have been, and a jet-black colour.
"The dig leader was able to tell me almost straight away what it was."
Palaeontologist Martin Simpson, who runs the fossil exhibition at the Black Gang museum on the Isle of Wight, said: "It certainly is an unusual find. While we have a couple of specimens of Valdosaurus bones in our own collection, there are no examples of a complete skeleton.
"Daniel is very lucky to have found a bone like this on a holiday fossil hunt. The Valdosaurus isn't a very well-known dinosaur."
Daniel said he will keep the bone for a while to show his friends before lending it to the museum to put on public display.

Valdosaurus was a plant-eating dinosaur which lived during the early Cretaceous period, between 120 and 125 million years ago.
It inhabited Europe and Africa, and grew to over three metres in length.