A new cathode material, K2V3O8, is reported reports as a new active material for potassium storage. The K2V3O8 is stabilized into tetragonal structure with P4bm space group, and we verify the unique crystal structure that is coordinated with four hendecahedra of KO11, VO5, and VO4, in which one VO5 shares its four oxygens with four VO4 tetrahedra to produce four penta-shaped voids that allows insertion and extraction of potassium ions. The K2V3O8 modified with carbon exhibits a reversible capacity of 75 mAh (g-oxide)(-1), and its electrochemical activity originates from the reversible de-/intercalation of potassium ions into/out of the hendecahedrally coordinated K2V3O8-carbon composite cathode, accompanied by a reversible V5+/4+ redox pair, which is retained for 200 cycles with 80% retention of its initial capacity.