Stoichiometric 4H-SrMnO3.0 nanoparticles have been synthesized from thermal decomposition of a new molecular heterometallic precursor [SrMn(edta)-(H2(O))(5)]center dot 3/2H(2)O whose crystal structure has been solved by single crystal X-ray diffraction. From this precursor, highly homogeneous 4H-SrMnO3.0 nanoparticles, with average particle size of 70 nm, are obtained. The agglomeration of these nanoparticles maintains the sheet-assembling morphology of the metal organic compound. Local structural information, provided by atomically resolved microscopy techniques, shows that 4H-SrMnO3.0 nanoparticles exhibit the same general structural features as the bulk material, although structural disorder, due to edge dislocations, is observed. The nanometric particle size enables a topotactic reduction process at low temperature stabilizing a metastable 4H-SrMnO2.82 phase. The oxygen deficiency is accommodated through extra cubic layers breaking the ...chch... 4H-sequence. These defect areas are Mn3+ rich, as evidenced by high energy resolution EELS data. Magnetic characterization of nano-SrMnO3.0 shows significant variations with respect to the bulk material. Besides the dominant antiferromagnetic interactions, a weak ferromagnetic contribution as well as exchange bias and a glassy-like component are present. After the reduction process, the stabilization of Mn3+ in the 4H-structure gives rise to magnetic anomalies in the 40-60 K temperature range. The origin of such magnetic features is discussed.