Physical Unclonable Function (PUF) has emerged as a hardware security block designed with low-cost and key generation for IC identification and authentication. The process variations being uncontrollable, they can be exploited as PUF that could generate unique identifiers representing robust keys. Arbiter-based PUFs work on the principle of the conventional delay-based approach realized between two symmetrical engaged paths. On the other hand, oscillator-based PUFs work on frequency differences among a group of identical oscillators arranged in a specific pattern. In this paper, a novel PUF is proposed based on Relaxation Oscillator PUF (ReOPUF) topology for device identification and authentication that can produce unique, unpredictable, and reliable keys to improve the robustness against the supply voltage and temperature variations. The ReOPUF is designed to generate a 4.4 MHz frequency that is suitable for powering IoT sub-systems including sensors while protecting them from malicious attacks. Based on Monte Carlo simulations, the reliability of PUF responses has been improved from 95.33% for the regular Ring Oscillator (RO) PUF to 99.19% for the proposed ReOPUF over a temperature range of $$-40\,^\circ $$-40∘C to +120 $$^\circ $$∘C with ±10% fluctuations in supply voltage. Moreover, it achieves a good uniqueness result of 49.22%, diffuseness of 49.52%, and worst-case reliability of 97.41% over a range of 10 $$^{\circ }$$∘C to 85 $$^{\circ }$$∘C, and 10% fluctuations in supply voltage. Thus, we report significant improvement over previous works.