Lund, 59, pleaded innocent but was found guilty involuntary manslaughter. The case has been dubbed "The lady in the lake" by the British press.
The family of his wife, Evelyn Lund, broke down in tears when the verdict was announced.
They had accused Robert Lund of abusing his wife and plotting to take the money she had inherited from a previous marriage.
Chief prosecutor Jean-Louis Bec told the court in Montauban that Lund had killed his wife and stuffed her blood-covered body into her car before pushing it into Lake Bancalié.
Lund, a British tree surgeon, was on trial in southern France for the third time, having appealed against his 2007 conviction for involuntary manslaughter.
Lund reported his 52-year-old wife Evelyn missing on New Year's Day 2000, telling investigators he believed she had an accident after drinking heavily and setting off to visit friends.
Her body was found in her car two years later when a drought lowered the water level of the lake.
The Lunds moved to the Tarn region, home to a large British expatriate community, in 1997 and rebuilt a farmhouse in the village of Rayssac.
Lund told the court that "a charge of murder does not mean a murder has taken place … all you have heard are speculations and lies which all have their roots in family emotions".