We propose a system for measuring entropy variations, S, in thermal systems using pyroelectric sensors. These sensors convert time-dependent temperature variations into electrical current. Consequently, heat and temperature variations are obtained, and sensor entropy is inferred. Various polyvinylidene fluoride and lead zirconate titanate sensors have been tested. Two types of measurements are performed. One in the volume, to measure entropy variations in a heat source, and the other on the surface, to measure entropy flux delivered by the heat source. Thermodynamic models and heat transfer dynamic simulations agree with the experimental results and relate the sensor entropy to the heat source entropy. These results show that pyroelectric sensors can enable entropy monitoring of thermal processes to improve system performance.