Thermostatic mixing valve for a hydraulic circuit, comprising: - a valve body provided with a first connection for the entry of a first flow of fluid in a first direction; - a second connection connected to said valve body for the entry of a second flow of fluid in a second direction arranged transversely to said first direction; - a third connection for the exit of a third flow, in particular of mixed fluid, in a third direction arranged transversely to said second direction; - a plug element that is slidable inside a housing cavity defined within said valve body and said second connection, on said plug element there being obtained passage opening means for said first flow and for said second flow; - a closing portion cooperating with said passage opening means to define a first passage port for said first flow of fluid and a second passage port for said second flow of fluid - thermostatic actuating means and counteracting elastic means arranged for driving said plug element and varying in a controlled manner said first passage port and said second passage port so as to maintain constant, at a desired value, the temperature of said third flow of fluid, said plug element comprising a tubular body bounded externally by a cylindrical surface on which said passage opening means for the fluid is circumferentially obtained, said closing portion being defined by an annular closing-sealing-element housed in a seat obtained in said valve body and projecting radially so as to intercept said passage opening means for the fluid, said closing-sealing-element comprising an operating part arranged in a cantilevered manner and having an internal bounding surface, which faces an internal zone of the plug element, and edge zones (22A, 22B) mutually spaced, wherein said internal bounding surface and said edge zones (22A, 22B) are at least partially positioned inside said passage opening means, CHARACTERISED IN THAT said plug element is made of a single piece and said passage opening means comprise one or more slits obtained through the thickness of a bounding wall of said plug-element, AND IN THAT said annular closing-sealing element is made of an elastically deformable material and the external cylindrical surface of said plug-element has an external diameter which is greater than a minimum internal diameter of said edge zones (22A, 22B), said operating-part centripetally projecting towards said plug-element and penetrating at least partially inside said one or more slits. - a valve body provided with a first connection for the entry of a first flow of fluid in a first direction; - a second connection connected to said valve body for the entry of a second flow of fluid in a second direction arranged transversely to said first direction; - a third connection for the exit of a third flow, in particular of mixed fluid, in a third direction arranged transversely to said second direction; - a plug element that is slidable inside a housing cavity defined within said valve body and said second connection, on said plug element there being obtained passage opening means for said first flow and for said second flow; - a closing portion cooperating with said passage opening means to define a first passage port for said first flow of fluid and a second passage port for said second flow of fluid - thermostatic actuating means and counteracting elastic means arranged for driving said plug element and varying in a controlled manner said first passage port and said second passage port so as to maintain constant, at a desired value, the temperature of said third flow of fluid,