The marble slab, which is just over 60cm (24in) high, is inscribed in an early Hebrew script called Samaritan.
It only includes nine of the original Biblical commandments; the 10th is a local Samaritan rule.
It was sold at a Los Angeles auction house on condition that it will be displayed in a public museum.
The carving was discovered in 1913 during the excavation for a railway line near the city of Yavneh, an historical Jewish centre of learning, now in present-day Israel.
The auctioneers, Heritage Auctions, said it probably dates from between 300 and 500 AD, and could have marked the entrance to a synagogue.
The missing commandment - "Thou shalt not take the name of the Lord thy God in vain" - has been replaced with call for Samaritan worshippers to "raise up a temple'' on Mount Gerizim, a holy site above the city of Nablus, in the present-day West Bank.
The buyer has not been identified.
The seller, Rabbi Shaul Deutsch from New York's Living Torah Museum, insisted that the new owner display the ancient artefact publicly, according to Heritage Auctions.
The Samaritan community has a long history in the Middle East, although its population has dwindled to fewer than 1,000 people in recent times.
The US auctioneers provided a translation of the 20 lines of Samaritan text on its website:
The world's earliest-known stone carving of the Ten Commandments has sold in a US auction for $850,000 (Â£680,000).