An under-sink/under-washbasin thermostatic mixing valve provides a first cold water inlet, a first cold water outlet, a second hot water inlet, a second mixed cold/hot water outlet; a thermostatic member and a shutter operated by the thermostatic member to ensure that the mixed water is kept at a predetermined temperature. A hollow insert is received along the first cold water inlet and is provided with longitudinal fins on the outer wall thereof to obtain two separate coaxial passages, for sending the cold water both directly to the first cold water outlet and to the area for mixing it with the hot water through the shutter to supply the mixed water to the second hot water outlet. In this way a compact and inexpensive thermostatic mixing valve is obtained.