The markings include Christian symbols, Greek letters and Latin writing and were found on a stone that is approximately 60cm.
Experts from the Cornwall Archaeological Unit, made the discovery at Tintagel castle and they believe the scribe was practising writing.
English Heritage Curator Win Scutt described the findings as “incredible”.
“We can't know for sure who made these marks or why, but what we can say is that 7th century Tintagel had professional scribes who were familiar with the techniques of writing manuscripts – and that in itself is very exciting,” Mr Scutt said.
“The survival of writing from this period is rare and this is a very important find, especially in terms of the continuity of a literate Christian tradition in post-Roman Cornwall,” Michelle Brown, a writing expert from the University of London, said.
“The lettering style and language used, as well as Christian symbols exhibiting Mediterranean influence and contacts, all reveal precious clues to the culture of those who lived at Tintagel in the 7th century.”
The inscribed slate stone had the Latin words “fili” (son) and “viri duo” (two men) on it and also Roman and Celtic names – “Tito” (Titus) and “Budic”.
Tintagel castle has been described as the place of King Arthur’s conception and the site has been described as one of the most spectacular in Britain by English Heritage.