We report a novel photonic bandgap sensor, comprising a D-shaped fiber fabricated via side polishing. It takes advantage of measurand-elicited refractive-index profile changes upon exposure of its side-polished region to an ambient liquid. The bandgap shift can be induced by perturbations in close proximity to the fiber core, such as the change of refractive index in the surrounding. The side-polished all-solid photonic bandgap fiber utilized offers an infiltration-free sensing mechanism, which eliminates difficult air-hole infiltration and evacuation processes experienced by most photonic crystal fiber-based index sensors. Experimental results demonstrated the repeatability and linearity in refractive-index sensing from 1.31 to 1.43 with a sensitivity of 10 -5 refractive-index units (RIUs). Additional study was also conducted to identify the significance of polishing depths in relation to the sensor performance.