This article introduces a 36.6-kHz unregulated ring oscillator (RO) tailored for Internet-of-Things (IoT) applications, demonstrating robustness to variations in supply voltage and temperature. A temperature compensation scheme of gate–source voltage ratio is theoretically proven and shows that the potential to be applied in some CMOS technologies that lack resistors with complementary temperature coefficients (TCs). Combining PMOS-only delay cells along with a complementary-to-absolute-temperature (CTAT) current reference, the RO ensures stable frequencies within variation ranges of up to 2.4 V and 180 °C, while exhibiting low process variations. In addition, a current-reuse technique is employed to mitigate the resistor area. Fabricated in a 65-nm CMOS technology, the RO occupies an area of 0.012 mm2, and nine samples are measured. The prototype achieves an energy efficiency of 2.74 pJ/cycle and exhibits a supply sensitivity of 0.08%/V ranging from 1.3 to 3.7 V without voltage regulators. Moreover, thanks to OFF-state leakage currents for high-order compensation, a TC of 32.3 ppm/°C in the box method is achieved from -30 °C to 150 °C with frequency trimming at room temperature (RT) and batch trimming.