A molecule-based mixed conductor of proton and electron was synthesized by modifying a nickel(III) dithiolene complex with 1,4-naphthoquinone skeletons. The complex forms a pi-stacking column flanked by an infinite hydrogen-bonding chain of water molecules. Room-temperature (RT) proton conductivity was 7.2 x 10(-7) S cm(-1), which is derived from protons attached to carbonyl oxygens through the chain of water molecules. The compound also shows a relatively high RT electronic conductivity of 4.1 x 10(-3) S cm(-1) and semiconducting behavior with a small activation energy of 66 meV, which originates from the pi-stacking column composed of the neutral radical complexes.