Metal oxides are promising for photoelectrochemical (PEC) water splitting due to their robustness and low cost. However, poor charge carrier transport impedes their activity, particularly at low-bias voltage. Here we demonstrate the unusual effectiveness of phosphorus doping into bismuth vanadate (BiVO) photoanode for efficient low-bias PEC water splitting. The resulting BiVO photoanode shows a separation efficiency of 80% and 99% at potentials as low as 0.6 and 1.0 V, respectively. Theoretical simulation and experimental analysis collectively verify that the record performance originates from the unique phosphorus-doped BiVO configuration with concurrently mediated carrier density, trap states, and small polaron hopping. With NiFeO cocatalyst, the BiVO photoanode achieves an applied bias photon-to-current efficiency of 2.21% at 0.6 V. The mechanistic understanding of the enhancement of BiVO properties provides key insights in trap state passivation and polaron hopping for most photoactive metal oxides.