The Thérain () is a river in France, tributary of the Oise. It is long. It rises between Saint-Michel-d'Halescourt and Grumesnil in Seine-Maritime at 175 meters elevation. It flows generally southeast, through Songeons, Milly-sur-Thérain, Beauvais, Hermes and Mouy, and joins the Oise at Creil.
Its valley, near the metropolitan area of Paris, has been a highly industrialized and populated area, and Beauvais lies at the foot of wooded hills on the left bank of the Thérain at its confluence with the Avelon.