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We have investigated the magnetic properties of Mn_1/3NbS₂, through neutron diffraction and magnetic susceptibility measurements. Mn_1/3NbS₂ crystallizes in the non-centrosymmetric hexagonal space group P6₃22 and is built from NbS₂ layers intercalated by Mn ions. The Mn atoms are situated in the octahedral holes between trigonal prismatic layers of 2H-NbS₂. The magnetic susceptibility and magnetization indicate a magnetic transition at T_N ~ 45 K with an anisotropy that is similar to that observed in Cr_1/3NbS₂. Neutron diffraction reveals increased scattering near the structural Bragg peaks. However, our data indicate that the magnetic scattering had a wider q-dependence than the nuclear Bragg peaks suggesting a long wavelength modulation of the magnetic ordering along the c-axis.