Béatrix de Mauvoisin, Lady of Rosny

Béatrix de Mauvoisin (born c. 1331) was the second of three daughters of Guy VII de Mauvoisin, baron of Rosny, and Roberte de Beaumetz. In 1355 she married Jean III, seigneur of La Ferté-Fresnel in Normandy.

After her father died without a son, her uncle Amaury, a canon in Paris, sold her the castle and lordship of Rosny by a charter dated 29 April 1365. After Béatrix's death, Rosny passed to her younger sister Marguerite's son Robert V d'Harcourt.