This paper presents a fiuidic probe that can self-adapt its shape to teeth and detect dental caries by sensing the variation in electrical-impedance. The fiuidic probe, whose liquid tip spontaneously spreads on hydrophilic tooth surface and into underlying caries, is employed to create intimate contact for impedance sensing. A tubular air sleeve shaped by the probe casing is applied around the liquid tip to insulate it from surrounding saliva, and to regulate its spreading. In the prototype demonstration, un-restored, extracted premolar teeth were investigated and the results indicated >20-fold impedance differences between sound and carious teeth, by which caries can be identified in a consistent manner. Furthermore, the fluidic probe has been successfully applied to approximal caries, which is difficult to detect by most existing schemes.