The stone, thought to have been carved about 1,200 years ago, is decorated with a number of Pictish symbols and is said by experts to be of national importance.
It is believed the stone, discovered at an early Christian church site in Dingwall, originally stood at more than two metres high.
It now measures around 1.5 metres, having been broken over the years and been reused as a grave marker in the 1790s.
Anne MacInnes, from the North of Scotland Archaeological Society, was the first to recognise the stone while carrying out a survey at the church site.
She said: "I was clearing vegetation when I spotted the carving. I really couldn't believe what I was seeing."
The find was verified by archaeologists from Highland Council and Historic Environment Scotland, before being safely removed from the site by specialist conservators on Thursday.
It will now be professionally conserved with a view to ultimately putting it on public display at a Highland museum or other suitable venue.
Designs on the stone include several mythical beasts, oxen, an animal-headed warrior with sword and shield, and a double disc and z rod symbol.
Details of the carvings on the reverse side of the stone are not yet known but experts suggest that, based on examples from similar stones, they are likely to include a large ornate Christian cross.
It would make the stone one of an estimated 50 complete or near complete Pictish cross-slabs known across the world, and the first to be discovered on the Scottish mainland for many years.
John Borland, president of the Pictish Arts Society, said: "The discovery of the top half of a large cross slab with Pictish symbols is of national importance.
"The find spot - an early Christian site in Easter Ross - is a new location for such sculpture so adds significant information to our knowledge of the Pictish church and its distribution.
"This new discovery will continue to stimulate debate and new research."
Kirsty Cameron, an archaeologist at Highland Council, said: "This is a once-in-a-lifetime find and what started as a small recording project has resulted in the identification of not only this important stone, but also that the site itself must be much older than anyone ever expected.
"All credit goes to the local archaeologists for immediately recognising the importance of the stone and putting plans in place for securing its future."
Stay up to date with the latest news stories from a Christian perspective. Sign up to our daily newsletter and receive more stories like this straight to your inbox every morning.