The Lewis Chessmen were discovered in 1831 in the Outer Hebrides of Scotland. Regarded as among the finest medieval chess pieces, most are on display in The British Museum in London and the National Museum of Scotland in Edinburgh.
Some 93 pieces were found at the time, with five pieces missing. The piece to be auctioned July 2 is the first additional piece to be found.
Sotheby's Alexander Kader said Monday the find is "one of the most exciting and personal rediscoveries to have been made during my career."