In p/n photoelectrochemical (PEC) cell systems, a low onset potential for the photoanode, as well as a high photocurrent, are critical for efficient water splitting. Here, we report a Mg-Zr cosubstituted Ta3N5 (Ta3N5:Mg+Zr) photoanode, designed to provide a more negative onset potential for PEC water splitting. The anodic photocurrent onset on Ta3N5:Mg+Zr was 0.55 V-RHE under AM 1.5G-simulated sunlight, which represented a negative shift from the ca. 0.8 VRHE for pure Ta3N5. This negative shift in the onset potential of PEC water splitting was attributed to the change in the bandgap potential due to partial substitution by the foreign ions Mg2+ and/or Zr4+.