Oxynitrides are potential photoanodes that combine properties of reasonable stability like pure oxides, with low band gap in the visible range identical to nitrides. However, low carrier mobility restricts the solar-to-hydrogen conversion efficiency from the theoretical limit. We found that polarization potential created by lattice distortion around Ta generates a driving force to trap electrons and forms small polarons in tantalum oxynitride (β-TaON). The localized small electron polaronic state is more favourable than the bloch-like extended state. The donated electron from n-type single donor defects becomes self-trapped and forms a weakly bound state with the defect. The electron polarons show non-adiabatic thermally activated migration via nearest neighbour hopping. However, O substitution at bridging N site increases the Ta–Ta hopping distance and changes the polaron hopping toward an adiabatic regime. The calculated low polaron mobility because of high migration barriers explains the experimentally observed high carrier lifetime and transport property of the β-TaON photoanode. This study provides a fundamental understanding of the charge trapping and transport of small polarons in TaON. It also prescribes a strategy to search for potential dopants to improve photocurrent generation by boosting polaron transport.