In this paper, a novel pressure-sensing technique based on measuring the relative permittivity of air is introduced. Previous works in this area are mostly centered around mechanical sensors paired with electrical readout circuits. However, the proposed CMOS-based pressure sensor is able to achieve the full sensing process electrically, from the sensing itself to the readout, with no need for post-processing or mechanical parts. In order to sense the pressure-dependent permittivity, a comb-structured top metal layer in a standard-CMOS process is proposed. The layer's capacitance is read with a capacitance-to-voltage converter using a differential RC circuit. The proposed readout circuit addresses issues such as 1/f noise for a more accurate output. A prototype chip was realized with a 0.25-mu m standard-CMOS process, and consumed 1.3 mW using a 2.5-V supply. The chip testing results showed a correlation between pressure and output voltage of 6 mV per atm of pressure change, confirming the functionality of this technique.