Painted in Normandy in 1890, it has not been exhibited for more than 100 years.
Monet's Matinée sur la Seine, près de Giverny was the star lot at Christie's auction house in London and was sold for £3.2m.
But the Monet was upstaged by a world auction record for the Spanish cubist artist Juan Gris.
Gris' work Le Gueridon, painted in 1914, fetched £4.4m.
"This is the highest total for an impressionist and modern art sale held here in London for over a decade, which is a clear indicator of the quality of the works on offer and where the market stands at present," said Jussi Pylkkanen of Christie's.
A separate sale of post-war art raised £3.9m.
The top lot was an untitled piece by Mark Rothko, painted in 1968, which sold for £718,750.
In a rare coincidence, another scarcely seen Monet painting is available in a separate sale at Sotheby's on Tuesday.
Meules, Derniers Rayons de Soleil - usually known as Haystacks but which actually depicts grain stacks - could fetch up to £7m.
Painted in Normandy in 1890, it has not been exhibited for more than 100 years.
One other Monet painting, Au Parc Monceau, will also be sold - from which a proportion of the expected £1.5 to £2m takings will go towards a Jewish family that was forced to sell it by the Nazis in 1935.
It had belonged to Ludwig Kainer, a painter, illustrator and set designer who worked for the Ballet Russes and the Berlin opera, before one of a number of "Jew auctions".
A spokesman for Sotheby's declined to reveal what proportion of the money will go to Kainer's descendants and how much to the current owner, a European collector who wants to stay anonymous.
Paintings by Renoir, Van Gogh and Chagall will also form part of the Sotheby's auction.