In this study, targeting applications in flexible electronic devices, we developed and evaluated a capacitive humidity sensor with high sensitivity and excellent linearity, employing a polyimide (PI)/carbon nanotube (CNT) composite thin film containing uniformly dispersed CNTs at high concentrations. The humidity-sensitive layer was fabricated using a proprietary dispersion and film-forming method, whereas interdigitated comb-shaped electrodes were created through semiconductor microfabrication techniques. A series of sensors were fabricated with varying electrode geometries (characterized by the dimensionless parameters r and h) and CNT contents ranging from 0 to 20 wt%. The sensors were evaluated on the basis of their capacitive response to humidity, linearity, hysteresis, and response speed. The results showed that miniaturization of the electrode geometry improved sensitivity, and the optimal performance—characterized by high sensitivity, excellent linearity, and low hysteresis—was achieved at a CNT content of 10 wt%. Compared with the CNT-free sensor, the 10 wt% CNT-containing sensor exhibited a 14.4-fold increase in sensitivity and a 4.6-fold increase in the maximum capacitance change rate. These findings contribute to the development of humidity monitoring technologies suitable for integration into component-embedded flexible substrates and provide a fundamental basis for advancing flexible sensor technologies.