Magnetism in Mn1.25Sb is discussed using the combined results of measured structural and magnetic properties and spin-polarized density functional theory calculations. Mn1.25Sb crystallizes in NiAs-type hexagonal structure, where the Mn-atoms occupy the 2a-sites and 2d-sites. The lattice parameter values are a = b = 4.2134 (1) Å and c = 5.6890 (4) Å. From the dc-magnetization measurements, it was understood that the compound exhibits a typical R-type ferrimagnetic behavior. The total magnetic moment follows predominantly T3/2 behavior up to 60 K and predominantly T2 behavior above 90 K. The values of critical exponents (β = 0.60 ± 0.01, γ = 1.38 ± 0.04 and δ = 3.32 ± 0.01) indicate that the magnetization conforms to the mean-field theory as well as to the 3D- Heisenberg model with local or short-range magnetic interactions. From the DFT calculations, it was understood that the magnetic moment of Mn-atom at the 2d-site (μMn2d=4.09μB) is aligned antiparallel to the magnetic moment of the Mn-atom at the 2a-site (μMn2a=4.47μB). The exchange interaction strength parameter (J) is found to be positive for μMn2a-μMn2a and negative for the μMn2a-μMn2d confirming the presence of competing magnetic interactions in the system. © 2020