Anodically fabricated tantalum oxide (Ta2O5) nanorod array carpets are converted into the corresponding tantalum oxynitride (TaON) through nitridation in an ammonia atmosphere. The measured optical bandgap energy of TaON is ∼2.3 eV, which is also confirmed via the density functional theory calculations. When used to photoelectrochemically split water (AM 1.5G illumination, 1 M KOH, and 0.6 V applied DC bias), the multilayer nanorod films show visible-light incident photon conversion efficiencies (IPCE) as high as 7.5%. The enhanced photochemical activity is discussed in terms of the ordered one-dimensional morphology as well as the electron effective mass in TaON and Ta2O5.