Extracting tritium of parts-per-trillion-levels from water at room temperature was provided using a protonic manganese oxide with a spinel crystal structure under weakly acidic conditions. Indeed, using 0.48 g of the protonic manganese oxide powder led to the removal of 1.75 x 10(5) Bq of tritium in 20 min at room temperature from a test water (100 mL) that contained a tritium concentration of 5.6 x 10(6) Bq/L (i.e., 15.6 ng/L). The extraction capability of tritium significantly depended on the crystal structure of manganese oxides and the proton content in the spinel crystal structure.