A foam generating device for a high-pressure water gun has a supporting body defining part of a conduit, which extends along an axis and has a mixing chamber communicating with a lower inlet, and subject, in use, to a vacuum to aspirate a liquid from said lower inlet; the device has a first sleeve which is fitted around one end of the supporting body, is axially fixed, is rotatable and is provided with an end portion carrying a nozzle; such a nozzle defines the outlet of the conduit and is adjustable so as to vary the outgoing flow type between a fan-like jet and a solid stream jet; the device has, in addition, a second sleeve which is coupled to the first sleeve in a fixed axial position and in a rotatable manner and is provided with cams to adjust the nozzle in response to a rotation of the second sleeve with respect to the first sleeve.