A 0.45-V resistor-based temperature sensor for energy-harvested Internet-of-Things device is presented. The sub-0.5-V operation is achieved by introducing a digital-intensive frequency-locked loop to convert the temperature-sensitive resistance to timing information. To overcome the high on-resistance of the transistors at sub-0.5 V, we apply gate-bulk-connected transistors to improve the on-resistance while safeguarding their leakages in the off-state. Simulated in a 65-nm CMOS process, the temperature sensor consumes 3.3 μW at room temperature. With a 10-ms conversion time, the temperature sensor can achieve a resolution of 10 mK, resulting in a resolution FoM of 3.3 pJ•K2.