ABSTRACTStoichiometric 4H-SrMnO_3.0 nanoparticles have been successfully synthesized for the first time from thermal decomposition of a new heterometallic precursor [SrMn(edta)(H₂O)₅]·³/₂H₂O. From this precursor, highly homogeneous 4H-SrMnO_3.0 nanoparticles with average particle size 70 nm are obtained. Local structural information, provided by atomically-resolved microscopy techniques, shows that 4H-SrMnO_3.0 nanoparticles exhibit the same general structural features than the bulk material, although structural disorder, due to edge-dislocations, is observed. The nanometric size of particles enables a topotactic reduction process at low temperature stabilizing a metastable 4H-SrMnO_2.82 phase. The oxygen deficiency is accommodated through extra cubic layers breaking the …hchc… 4H-sequence. These defect areas are Mn³⁺ rich as evidenced by high energy resolution EELS data. Magnetic characterization of nano-4H-SrMnO_3-δ shows significant variations with respect to the bulk material.