The price, which includes buyer's premium, is the second highest ever for a Monet at auction, outstripping the $35.6 million paid Monday at Christie's for "Waterloo Bridge, Temps Couvert" ("Waterloo Bridge, Overcast Weather").
"Waterlily Pond and Path by Water" set a record for a Monet in 1998 when Sotheby's sold it at auction for $32.9 million -- more than $40 million in today's dollars.
"Nympheas," which has not been seen in public since 1936, is one of only a few of Monet's iconic water lily pictures still in private hands. It was sold by an anonymous French collector whose family had owned it for 80 years.
Melanie Clore, co-chair of Sotheby's impressionist and modern art department, said the painting was "by far the best work by the artist to have appeared on the market in recent times."
Experts estimate that this week's London sales at Sotheby's and rival Christie's could generate almost $1 billion.
Christie's sale of impressionist and modern art on Monday generated $240 million, the auction house said, a record for an auction in Europe.