Ex-mayor René Marratier was sentenced to four years imprisonment over the deadly flooding of the coastal town of La-Faute-sur-Mer in 2010.
The court ruled that Marratier was aware of the risks of flooding but "deliberately hid" them to avoid impeding the "financial windfall" from property development on the seashore.
According to the court, the building frenzy led to the death of 29 people - most of them elderly or children who were trapped and drowned in their homes - on the night of 28 February 2010.
Marratier's deputy Françoise Babin was sentenced to two years imprisonment and a fine of 75,000 euros.
She was in charge of building permits at the same time as being a property developer.
Her son, Philippe Babin, was sentenced to 18 months in jail for not ensuring the monitoring of the seawall.
Marratier said he will appeal against the sentence.