A self-referenced VCO-based temperature sensor with reduced supply sensitivity is presented. The proposed sensor converts temperature information to frequency and then into digital bits. A novel sensing technique is proposed in which temperature information is acquired by evaluating the ratio of the output frequencies of two ring oscillators, designed to have different temperature sensitivities, thus avoiding the need for an external frequency reference. Reduced supply sensitivity is achieved by employing the voltage dependence of junction capacitance, thus avoiding the overhead of a voltage regulator. Fabricated in a 65 nm CMOS process, the prototype can operate with supply voltages ranging from 0.85 V to 1.1 V. It achieves supply sensitivity of 0.034 °C/mV and an inaccuracy of ±0.9 °C and ±2.3 °C from 0 to 100 °C after 2-point calibration, with and without static nonlinearity correction, respectively. The proposed sensor achieves 0.3 °C resolution, and a resolution FoM of 0.3 nJK2. The prototype occupies a die area of 0.004 mm2.