The find, which has been described by experts as “tremendously exciting”, was made during restoration works to the Lochaber castle.
The cannonball was found sitting in an 18 inch hole in one of the castle’s sea facing walls, embedded a foot into the wall.
It is believed the weapon was fired from a ship during a famous siege of the castle in 1644.
A vessel thought to be of Dutch origin was lost in the attack on the castle during tensions between clans and Covenanters.
The remains of the ship were discovered in 1999 and relate to the siege of Mingary Castle by by Archibald Campbell, 8th Earl of Argyll, the castle’s former owner, recorded in a diary written by John Weir in 1644.
Weir was a Puritan imprisoned in Mingary Castle by Major-General Alasdair MacDonald. Five guns have been found on the seabed in association with other scattered, wreck-related material.
The discovery was made after scaffolding was erected around the high regions of castle walls.
Jon Haylett, 68, a local historian who is working on the renovation, said: “It is a tremendously exciting find because, as with the blackened gate stones which were found in the moat, it can be traced back to a particular event in Mingary Castle’s history.
“Since this wall faces out to sea, the cannon that fired it must have been aboard a ship. It’s possible that this dates to the siege of 1644 when one of the ships investing the castle, a Dutch vessel, sank just off the castle.”
The ship wreck has been described as a site of “significant historic and archaeological importance” by Historic Scotland.
A report by the conservation organisation about the wreck states: “The site retains the potential to add knowledge to our understanding of the design and operation of naval activity in the Western Isles during the 17th century.
“When considered in the context of Duart Castle and the two wrecks associated with attacks-against it, the wreck off Mingary Castle adds to our understanding of the coastal landscape of the Sound of Mull and the growing vulnerability of castles to attack by seaborne artillery.”
It is the second cannonball to be found at the site after another one along with a musket ball was found at the bottom of the castle moat.
Other significant finds include a barbed arrowhead, pottery, bones and glassware.
Archaeologists were also excited to discover the charred remains of the sandstone castle archway, which it is believed were destroyed in an attack by Alasdair MacColla MacDonald in 1645.
The well-preserved castle near the most westerly point of the British mainland was a stronghold for several clan chiefs and the Lords of the Isles, who ruled the Scottish islands.
The site is being excavated to pave the way for a renovation project which will see it transformed into modern residential accommodation.