A mixing valve having a first inlet port (4), a second inlet port (6) extending transverse to said first inlet port (4) and an outlet port (8). The inlet ports (4, 6) open out into a valve chamber (10) and the outlet port (8) branches off this valve chamber (10), inside which a movable valve element (12) is arranged, having an obstruction (32) inside the second inlet port (6), which protrudes from an inner circumference (36) of the second port (6) into an interior thereof. The obstruction (32) extends at least along a portion of the inner circumference (36), which portion of the circumference (36) is located on an inner side of the second inlet port (6) being distant from the first inlet port (4). A heating or cooling device includes such a mixing valve.