The coupled neutron diffraction and Mossbauer study of the layered oxide Sr3NdFe3O9 shows for the first time the possibility of anionic vacancies ordering in an oxygen-deficient related Ruddlesden−Popper- (RP-) type ferrite leading to an original intergrowth of brownmillerite and K2NiF4-type layers. The antiferromagnetic properties of this phase up to 500 K are also evidenced. These results open the route to the synthesis of new members in the series (Sr2Fe2O5)n·SrLnFeO4 and to the understanding of their reactivity with atmospheric water.