An ultrasonic temperature sensor is based upon the variation in the speed of propagation of a torsional wave in a material whose torsional modulus varies as a function of temperature. A transducer for producing a torsional wave is coupled through an accoustic waveguide to a sensor element which has a torsional modulus which varies with temperature. A discontinuity in the sensor causes a portion of the torsional wave to be reflected while the remainder traverses the sensor and is reflected. The time delay between the first and second pulses is thus a measure of the temperature.