When tested as the cathode material for potassium-ion batteries, K2V3O8 delivers a highly reversible capacity of over 100 mA h g-1 in the voltage range of 1.5-4.5 V with 73% capacity retained after 50 cycles. Charge-discharge is achieved by a nontopotactic process between K2V3O8 and KV3O8 (ΔV ∼ -23.4%), causing the slow capacity decay.