A composite active waveguide temperature sensor (10) incorporates a first, sensor portion (16) formed of an environment-resistant material such as ceramic coupled through an ultrasonically-transparent bond (20) to a second, waveguide portion (18) formed of an ultrasonically-transmissive material such as a metallic filament wire. By doing so, the sensor portion (16) may be positioned within a harsh environment and subjected to a temperature to be measured, and the waveguide portion (18) may be used to propagate ultrasonic energy to and/or from the sensor portion (16) to a location distal from the harsh environment for measurement of the temperature. The ultrasonically-transparent bond (20) between these portions (16, 18) limits attenuation of and the introduction of reflections and other noise to an ultrasonic signal propagated across the bond (20).