After fierce bidding, Sur les Planches de Trouville went under the hammer for pounds 3.8m.
The unknown buyer bid from the floor at Christie's auction house in London, and had to fight off two telephone bidders in the "fast and furious" sale, according to a Christie's spokeswoman.
Monet finished the painting, which has dozens of sand particles stuck to the paint, in 1870. It has not been seen by the public for more than 100 years, having been bought in France in 1884 by the grandfather of last night's seller.
Monet, who lived from 1840 to 1926, was struggling to gain acceptance for his work at the time and took his new wife and son to Trouville on the Normandy coast to escape creditors in Paris.
Christie's said the painting was one of the best examples of his work seen at auction for more than a decade. The buyer will eventually pay pounds 4,181,500 including buyer's premium.Reuse content