This article describes an ultralow-voltage (ULV) resistor-based temperature sensor for sub-0.5 V energy-harvesting Internet-of-Things (IoT) devices. The key features are: 1) a digital-intensive frequency-locked loop (DFLL) with a swing-boosted RC front-end to enable ULV operation with high accuracy and avert the analog-to-digital converter; 2) a sample-and-boost dynamic comparator featuring background offset calibration and voltage regulation to safeguard the operation and accuracy at a sub-0.5 V supply; and 3) a resistor-regulated digital-controlled ring oscillator (DCRO) to extend the operational temperature range. Prototyped in 65-nm CMOS, the temperature sensor occupies 0.0294 mm2 and consumes 10.4 $\mu \text{W}$ under 0.4 V at room temperature. The resolution figure-of-merit (FoM) is 385 $\mathrm {fJ}\cdot \mathrm {K}^{2}$ , and the inaccuracy measures ±0.24 °C (p2p) over –40°C to 125 °C, which outperforms the state-of-the-art sub-0.5 V temperature sensors.