Here a novel water-containing derivative phase (with the elongated c axis from 20.2 to 28.0 Å) is reported for the n = 2 member of the Sr_n+1Fe_nO_3n+1-δ Ruddlesden−Popper series, i.e., Sr₃Fe₂O_7-δ with the layer sequence of SrO−SrO−FeO₂−SrO_1-δ−FeO₂. The intercalated water molecules in the derivative phase are believed to reside within the (SrO)₂ double-layer block, probably accompanied by additional protons. The latter belief is supported by the fact that upon water incorporation iron was found to be reduced (toward the trivalent state). High-quality samples of the parent Sr₃Fe₂O_7-δ phase were synthesized from a solution-derived precursor powder. For samples synthesized in air cerimetric titration yielded an oxygen content at 7 − δ = 6.44. More oxygen-deficient samples with 7 − δ = 6.32 and 6.18 were obtained by postannealing the as-synthesized material in flowing Ar gas at 400 and 500 °C, respectively. Independent of the value of δ, all the Sr₃Fe₂O_7-δ samples transformed to the water derivative when subjected to an ambient (humid) atmosphere, whereas the speed of the topotactic derivative-phase formation increased with increasing δ. On the other hand, reversible dehydration of the derivative phase occurred upon heating in two distinguishable steps below ∼230 °C.