A limitation currently facing internal combustion engine research and development is the lack of a direct method to accurately measure in-cylinder temperature. The rate at which an engine cycle evolves is too rapid for conventional, direct measurement transducers such as thermocouples or thermistors. This paper presents the experimental results of a novel method for determining time-resolved in-cylinder temperature using ultrasonic thermometry. The technique involved sampling an ultrasonic signal reflected from the top of the moving piston and measuring piston position using an optical encoder connected to the engine crankshaft. The known flight distance and measured time of flight (ToF) was used to determine path-averaged temperature. ToF of the ultrasonic signal was precisely determined using an unscented Kalman filtering technique. Experiments were conducted using a motored (non-combusting) engine without compression at two engine speeds and three known intake temperatures. Results show that the method is capable of measuring temperature to within an accuracy of 10%. Repeated temperature samples over consecutive cycles had standard errors below 0.25% when a significant number of samples were available to analyze. Overall, our work proves that path-averaged, in-cylinder temperature measurement using ultrasonic thermometry is a feasible approach for use in engine research applications. With the availability of robust transducers that can withstand high temperature and pressure, we expect that the developed method can be applied to firing engines.