Na₂Fe₂Se₂O with the I4/mmm space group was studied in sodium-ion batteries, delivering a reversible specific capacity of more than 140 mA h g⁻¹. Operando XRD and XAS studies disclosed bifunctional redox behaviour with the prevalence of anionic electrochemical activity and a likely partial decomposition of the material, which, however, does not influence the electrochemical behaviour of the system.