The Lock, finished in 1824, is the fifth in a celebrated series of six large-scale paintings of the Stour Valley that Constable exhibited between 1819 and 1825.
The painting, remarkable for its excellent state of preservation and from the collection of Baroness Carmen Thyssen-Bornemisza, sold tonight at Christie's in London.
The Lock was bought at the 1824 Royal Academy exhibition by James Morrison.
Born the son of an innkeeper, Morrison rose to become one of the wealthiest British merchants of the 19th Century and an outstanding collector of old and modern masters.
The Lock remained in the possession of his descendants until acquired by the current owner in 1990.
The auction house said the series of paintings defined Constable's artistic maturity and "represent a distillation of his profound emotional and artistic response to the scenery of his native Suffolk".
Among the series are several of the artist's most renowned works, including The White Horse (1819; New York, The Frick Collection), and The Hay Wain (1822; London, National Gallery).