Ventilation valve for a liquid tank comprising: an housing comprising a side wall provided with ventilation openings, a cover provided with a ventilation aperture which may be connected to a venting circuit of the tank, and a base pierced with at least one drainage aperture; and a float capable of closing off the ventilation aperture, such float being able to slide vertically inside the housing depending on the level of liquid therein, the valve also comprising a drainage chamber located underneath the housing in which the valve slides, the chamber also comprising at least one drainage aperture that is not aligned with the drainage aperture of the base of the housing.