Abstract: The non-therapeutic cosmetic treatment method includes at least: —applying an active composition including at least niacinamide onto the skin of a subject, and—illuminating the skin using at least one light source emitting at a wavelength of between 620 and 690 nanometres, preferably 660 nanometres, the illumination being carried out before, at the same time as or after the application of the composition. A device is configured for implementing such a method.