Perovskite-type lanthanum manganite (LaMnO3) was synthesized from Mn2O3 and La2O3 precursor powders using the Spark Plasma Sintering technique (SPS). The in situ reaction proceeds in a few minutes at 1000 °C under 50 MPa with a very high heating rate of 100 °C/min. The Rietveld refinement of the X-ray powder diffraction shows that LaMnO3 sample crystallizes in a perovskite structure of O′-type orthorhombic symmetry with Pbnm space group. After annealing at 1000 °C in air for 10 hrs, the symmetry changes to rhombohedral with R-3c space group. For the as prepared sample, two critical temperature points in the temperature dependence of the ZFC and FC magnetization occurring at 125 K and 145 K are observed. They are suggested to be related to A-type antiferromagnetic Néel temperature and ferromagnetic short range order, respectively. The ferromagnetic component in the annealed sample increases indicating the expected variation in the concentration of Mn3+ and Mn4+ ions, which was attributed to the oxygen stoichiometry effects.