A fluid circulation valve has a body delimiting an internal duct and a flap mounted pivotally on a spindle separating the flap into a first part and a second part. The flap is able to pivot between an open position allowing the fluid to pass into the duct, and a closed position which prevents the passage and for which the second part of the flap comes to rest against a seal which extends facing this second part, and which further seals the body of the valve. The seal has pressure-lowering means downstream of the second part of the flap so as to increase the force for clamping the second part against the seal.