Small fully inkjet-printed gas sensors based on capacitive parallel-plate (PP) structures have been realized on flexible plastic foil and characterized. A gas sensing layer was inkjet-printed between inkjet-printed bottom and top silver electrodes. Compared with comb electrode (CE) geometries, PP structures drastically reduce the developing complexity of gas sensors on polymeric foil, avoiding the substrate parasitic signal. Furthermore, the use of porous inkjet-printed metal makes the patterning of complex grids on the top electrode unnecessary, since such porosity permits the analyte to flow into the sensing layer. This low demanding patterning resolution facilitates the miniaturization of the inkjet-printed sensors, introducing significant improvements in their sensing performances, such as sensitivity or response time. The printed sensing devices were characterized against pulses of relative humidity (R.H.) and their performances were analyzed.