Piezoelectric sensors based on acoustic waves propagating into micro electro-mechanical systems (MEMS) are a lively research area, with devices based on flexural plate waves (FPW) finding applications across various fields, including industrial and biological sectors. In this paper, a temperature sensor based on a piezoelectric MEMS that exploits the first antisymmetric mode, denoted as the A0 or FPW mode, of Lamb waves generated in a diaphragm is presented. The micromachined diaphragm is composed of a stack of doped silicon and an aluminum nitride (AlN) piezoelectric layer with aluminum interdigital transducers (IDTs) employed for wave generation and detection. The proposed MEMS sensor exploits the variation of the FPW propagation velocity in the diaphragm due to temperature. The sensor expected working temperature range is at least 20 to 80 °C and in this preliminary phase has been tested from 25 to 45 °C by means of a tailored PID controlled temperature chamber including a Peltier cell and a Pt100 sensor while measuring the electrical admittance of a single IDT. The working principle has been validated within the frequency range from 11.185 to 12.88 MHz. Measurements have confirmed the sensor working principle, demonstrating a sensitivity of -426.5 Hz/°C and highlighting its potential for extending the operating temperature range.