Diffuser device for cosmetic liquids for hair dryers, characterized in that it comprises a cylindrical barrel, which can be associated with the blowing part of a hair dryer; said cylindrical barrel housing a diaphragm comprising a transverse septum which has a central expansion and lateral lobes; said lateral lobes defining respective lateral passages; said lateral passages and said transverse septum causing the air produced by said blowing part to pass in the form of two flows channeled to the exit of said device with laminar motion. The device makes passage of the air that passes through it more uniform in pressure and temperature, and at the same time enables cosmetic products to be dispensed for hair treatment.