The Vexin is a former region in France, divided since the 10th century between the Norman Vexin (Vexin normand) and the French Vexin (Vexin français).

The Gaulish tribe of the Veliocassi , whose capital was at Rouen, gave their name to the region that became known as the Vexin, later to become a county. The Norman incursions under Rollo of Normandy were halted by the Treaty of Saint-Clair-sur-Epte in 911 which established the Duchy of Normandy and fixed its boundary with the territory of the Kingdom of France along the river Epte. This divided the county of Vexin into two parts:

• the Norman Vexin, which became part of the Duchy of Normandy and is now part of the French region of Haute-Normandie. It is bounded by the rivers Epte, Andelle and Seine.

• the French Vexin remained part of the Île-de-France. It straddles the current départements of Val-d'Oise and Yvelines and is bounded by the rivers Epte, Oise and Seine. A regional nature park was established in the French Vexin in 1995.