From BBC News
A 50kg (7st 9lb) silver bar was brought to shore on Thursday on the island of Sainte Marie, from what is thought to be the wreck of the Adventure Galley.
The bar was presented to Madagascar's president at a special ceremony. US explorer Barry Clifford says he believes there are many more such bars still in the wreck.
Capt Kidd was first appointed by the British authorities to tackle piracy but later became a ruthless criminal and was executed in 1701.
"Captain's Kidd's treasure is the stuff of legends. People have been looking for it for 300 years. To literally have it hit me on the head - I thought what the heck just happened to me.
I really didn't expect this," Mr Clifford said. "There's more down there.
I know the whole bottom of the cavity where I found the silver bar is filled with metal. It's too murky down there to see what metal, but my metal detector tells me there is metal on all sides."
The BBC's Martin Vogl tweets that there is much excitement in Madagascar about the discovery and Mr Clifford's team has no doubt that the discovery is genuine.
The team believes the bar, marked with what appears to be a letter S and a letter T, has its origins in 17th-Century Bolivia.
It believes the ship it has found was built in England, however there is bound to be scepticism and calls for more proof that the bar was linked to Capt Kidd, our reporter says.