This paper presents the first reported CMOS 2-in-1 relative humidity (RH) and temperature (Temp) sensor. A unified analog front-end (AFE) interfaces two on-chip transducers and converts the resistance (a Temp) and capacitance (a RH) into a frequency. This conversion occurs with high linearity (±10 ppm error) across the industrial temperature range (-40 to + 85°C) via a frequency-locked loop (FLL). An incomplete-settling switched-capacitor (SC)-based Wheatstone bridge (WhB) is proposed to sense the Rand C in a power-efficient fashion. The FLL output is digitized by a low power, time-to-digital converter (TDC) that has high resolution due to the inherent quantization noise-shaping. This system achieves excellent resolution figure-of-merits (FOMs) when normalized to RH (0.83pJ·(%RH)²) and temperature (62 fJ·K²).