Bec-de-Mortagne (French pronunciation: [bɛk də mɔʁtaɲ]) is a commune in the Seine-Maritime department in the Normandy region in northern France.

A farming village situated in the wooded valley of the river Ganzeville, in the Pays de Caux, some 19 miles (31 km) northeast of Le Havre, at the junction of the D69 and the D28.

The name comes from ‘’Becr’’, a Norse word for stream, together with the name of the Mortagne family, seigneurs of the village.

Bec-de-Mortagne in the Pays de Caux is thought to be the birth-place of Turstin FitzRolf, standard bearer to William the Conqueror at the Battle of Hastings in 1066, as he was described by the 12th-century chronicler Orderic Vitalis as from "Bec-en-Caux".

The year 1562 saw the formal civil status of Bec de Mortagne granted and in 1824 it was joined with the former parish of Baigneville to form the present commune.