A flexible, transparent iontronic film is introduced as a thin-film capacitive sensing material for emerging wearable and health-monitoring applications. Utilizing the capacitive interface at the ionic-electronic contact, the iontronic film sensor offers a large unit-area capacitance ( of 5.4 mu F cm(-2)) and an ultrahigh sensitivity (of 3.1 nF kPa(-1)), which is a thousand times greater than that of traditional solid-state counterparts.