New York diamond cutter Isaac Wolf had outbid three rivals for the gemstone, called the Pink Star, but has since failed to pay.
"Sotheby's has reserved all of its rights and remedies against the defaulting purchaser," the auctioneers said in a statement.
They had guaranteed the vendor $60m for the stone that was listed in their annual report with an inventory value of $72m. The diamond was originally mined in South Africa in 1999 as a 132.5-carat 'rough' by De Beers.
Sotheby's still hold the world-record price for a gem stone, even after the default. In November 2010 the auction house sold another pink diamond, the 24.78-carat Graff Pink, for $46m.