A highly sensitive and fast responsive relative humidity (% RH) sensor based on In–SnO₂ loaded cubic mesoporous graphitic carbon nitride (g-C₃N₄) has been demonstrated in this study. The mesoporous In–SnO₂/meso-CN nanohybrid was synthesized through template inversion of mesoporous silica, KIT-6, using a nanocasting process. Due to its 3D replicated cubic structure with ordered mesopores, the nanohybrid facilitates the process of adsorption, charge transmission and desorption of water molecules across the sensor surfaces. Consequently, the optimized In–SnO₂/meso-CN nanohybrid exhibits excellent response (5 orders change in impedance) in the 11–98% RH range, high stability, negligible hysteresis (0.7%) and superior real time % RH detection performance. Compared to traditional metal oxide based resistive sensors with unique mesoporous/hierarchical/sheet-like morphology, the 3D mesostructured In–SnO₂/meso-CN nanohybrid demonstrated a superfast response (3.5 s) and recovery (1.5 s) in the 11–98% RH range at room temperature. These results open the door for breath monitoring and show a promising glimpse for designing mesoporous 2D layered materials in the development of future ultra-sensitive % RH sensors.