The New York auction house had expected it to sell for $1-2m.
The tablet was said to be discovered during excavations for the construction of a rail line.
It carries a Paleo-Hebrew script, and, according to Sotheby’s, was held privately until an archaeologist living in Israel realised its importance and purchased it.
“It’s been thrilling to work with this object of antiquity. There is no other stone like it in private hands,” Sharon Liberman Mintz, a specialist on Jewish texts for Sotheby’s, told AFP.
The slab eventually made its way to the Living Torah Museum in Brooklyn before being sold to a private collector.
In a statement, Sotheby’s said the tablet has been studied “by leading scholars in the field and published in numerous scholarly articles and books”.
However, multiple experts told the New York Times they had questions about its origins.
“Maybe it’s absolutely authentic,” said Brian Daniels, of the Penn Cultural Heritage Center in Philadelphia, though he cautioned: “Objects from this region of the world are rife with fakes.”
“There is no way” the age of the inscription can be known, Christopher Rollston, a professor of Biblical and near eastern languages and civilisations at George Washington University, told the paper.
“We have zero documentation from 1913, and since pillagers and forgers often concoct such stories to give an inscription an aura of authenticity, this story could actually just be a tall tale told by a forger or some antiquities dealer.”
-Agence France-Presse