This paper reports a high-sensitivity and high-speed capacitive humidity sensor. A novel capacitive vertical parallel-plate (VPP) array had been monolithically integrated with an on-chip ring oscillator (RO) using the standard TSMC 0.18μm CMOS process and subsequent in-house post-processes. Resorcinol-formaldehyde (RF) aerogel, a new kind of moisture-sensitive polymer, was deposited and defined in columns in the VPP array. Measurements show that the typical fabricated device has a sensitivity of 0.566% capacitance change per percent-relative-humidity (%RH) and a response time of 6s. The monolithic integration of the humidity sensor with RO is also demonstrated to provide a sensitivity of 3.8 kHz/%RH. Further improvement in sensitivity is required by varying the operating frequency of RO.