A French silver coin has been found embedded in the keel of a medieval ship uncovered on the banks of the river Usk in Newport three years ago.
The coin is inscribed in Latin and has a cross on one face
The discovery of the 15th Century coin is being interpreted as a sign that the ship came originally from France.
Experts believe the coin was new and was intended to be a good luck charm.
Project leader Kate Hunter said a colleague was shaking when she found the coin. She said: "We all understood immediately how important it was."
The Newport ship is the most complete surviving 15th Century vessel discovered in recent years. It was found on the banks of the river during the construction of The Riverfront arts centre.
The coin, wrapped in tarred caulking, was discovered in one of the wooden timbers being studied by the city council's ship recording team.
The ship is one of the most important finds of recent times
It was in a hole cut above the ship's keel at the point where it connects to the stem-post, the timber which forms the bow.
The two timbers would have been the first the ship's builders put in position and archaeologists believe the coin was inserted as a good luck charm.
Ms Hunter said colleague Angela Karsten was the first to spot the coin.
She said: "We think it's been put in as a good luck charm. There's a long tradition in ships of putting coins under the mast or in the keel as a good luck charm."
The coin has been identified by expert Edward Besly from the National Museum of Wales as a petit blanc of the Dauphin Louis de France, who became Louis X1 in 1461.
Minted in the town of Crémieu between 1440 to 1456, the coin comes from Dauphiné, an area of south-eastern France traditionally held by the Dauphin, the eldest son of the king of France.
The worker who made the discovery cleans the coin in the timber
The type of coin was in circulation through 15th Century France and the dolphin, the symbol of the Dauphin, is found on both sides.
The outward face of the coin shows a cross and is inscribed in Latin "Blessed be the name of the Lord".
The discovery also means the restoration team, who already know the ship was abandoned on wooden struts around 1468, can calculate the ship's working life.
The fact that many of the artefacts discovered in the vessel are European, experts say, points to the ship's continental origin.
The archaeological team is recording and analysing the 1,700 timbers in the ship using digital technology.
In December the restoration project received a grant of almost £800,000 from the Heritage Lottery Fund (HLF).
Ron Jones, cabinet member for culture and recreation on Newport council, said the latest find was "one of great significance".
Mr Jones said: "The whole restoration of the Newport ship is an exciting journey in itself, and I am sure this latest find is one of great significance.
"We look forward to many more discoveries as the project continues its excellent progress."