John I of Brienne, Count of Eu
John I of Brienne was the son of Alphonso of Brienne and Marie de Lusignan. His mother was the heiress of Eu, Seine-Maritime, and he succeeded his father as Count of Eu in 1260.
He married Beatrice, the daughter of Guy III, Count of Saint-Pol. They had:
- John II of Brienne, Count of Eu
- Isabelle, married John II of Dampierre, Viscount of Troyes
- Jeanne, married first Raymond VI, Viscount of Turenne, married second before 4 August 1314 Renauld, Lord of Picquigny, vidame of Amiens
- Marguerite, married Guy, Viscount of Thouars. Guy was the son of Aimery IX and Margaret of Lusignan, who was the daughter of Hugh X of Lusignan and Isabella of Angoulême.
- Mahaut, married Alfonso de la Cerda