In 1300 Edmund, Earl of Cornwall, died and the Manor of Trematon lapsed to the Crown. In 1337 Edward III created the Duchy of Cornwall for his eldest son, and the Manor passed into the possession of the new Duke. From the colour of his armour, Prince Edward, Duke of Cornwall (and, later, Prince of Wales), became known as the Black Prince. He did not outlive his father, and so never became king.

Saltash continued to prosper under the new regime. The annual fair held on "the Feast of St Faith with the Vigil" (5th and 6th October) was moved to a new site on the outskirts of the town, which saved having to drive the growing number of cattle through the streets to the central market area. A new chapel, dedicated to St Faith, was erected adjacent to the fairground, which was probably on the site of the present-day Regal Court. Fore Street and Middle Street (now Albert Road) were fringed on both sides with houses and workshops, and the northern side of Back Street (now Culver Road) was lined with properties, plus a few on its southern side. St Nicholas's Chapel was enlarged by the addition of a chancel, aisle and porch. The waterside had a thriving community, and built up an impressive reputation for ship building; during the Hundred Years War only Fowey produced more ships. Such was the growing importance of Saltash that it was chosen to hold the Assizes when the Black Death struck Launceston. Another measure of the town’s prosperity was the size of its rents, which were amongst the highest in the Duchy!

There is a tradition that the Black Prince, being at Saltash on the eve of a battle, wished to join his troops on the other side of the river, and that two Saltash women rowed him over to the Devon side but refused any reward. It is said that the Prince, in gratitude for this service, granted the Passage to the inhabitants of Saltash. This is a pleasant story, but it does not accord with the facts! In 1356 the Prince granted the revenues from Saltash Ferry, together with an annuity of £20, to his loyal servant and porter William Lenche, who had lost an eye serving him during the Battle of Poitiers earlier that year. In 1372 the "Manor of Trematon and the Passage of Asshe" was granted, again by the Black Prince, to Sir Nigel Loring. Richard II ratified Saltash's Charter in 1382, and it was he who, three years later, first granted "to the Mayor and Burgesses of our town of Saltash, in the County of Cornwall, that they themselves ..... should have the proceeds and revenues of a certain Ferryboat in our said town of Saltash, for a term of thirty years".

Saltash also developed as a port, the first to be established on the system of estuaries reaching inland from Plymouth Sound. The stretch of the Tamar bordering Saltash had the advantage of a safe deep-water anchorage for large ships at all states of tide, and Saltash ships were making long voyages to many destinations for a variety of reasons; wars with France, transporting pilgrims bound for the shrine at Compostela in northern Spain, and even trading within the Arctic Circle. As early as 1438 the Nicholas of Saltash, in defiance of a royal statute banning direct trading, ventured across the Arctic Circle to Finmark in the far north of Norway. Finmark and Iceland were then the most distant places reached by English ships, and it was a brave enterprise to take a small vessel to the Arctic Circle some 2,000 miles away. By the turn of the century Saltash vessels were to be exploring and fishing Newfoundland waters, even further distant.

Although by now eclipsed in size by Plymouth, Saltash had lost none of its local significance. In 1439 a Charter was granted to Plymouth agreeing to the fortification of the town, but only on the condition that it should not interfere with the Borough of Saltash or its rights over the Tamar! The rights of the ferry and most of the estuary system belonged to the Manor, but most of the water-based activities (operating the ferry, the fisheries, the oysterage, collecting dues for anchorage, buoyage, the barges, etc.) were leased to the burgesses of Saltash. Gradually these duties coalesced into one jurisdiction, known as the Liberty of the Water Tamar, and Saltash was regarded as holding it by prescriptive right, an arrangement which was challenged many times but was not finally terminated until the end of the nineteenth century.

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