A method for treating the hair with the aid of a treatment device, notably a heat treatment device, in particular a straightening device, in which a lock of hair is exposed, while the device is being moved along the latter, in a single operation to: apply cosmetic composition with the aid of an application member of the device, and then to apply steam with the aid of a steam outlet of the device that is offset relative to the application member in the direction of movement of the lock of hair by a nonzero distance v, measured between the median plane of the steam outlet and the median plane of the application member, of less than or equal to 10 mm.