Abstract: This work aims to study the improvement effect of Sm on Mn-based catalysts for selective catalytic reduction (SCR) of NO with NH₃. A series of Sm_xMn_0.3−xTi catalysts (x = 0, 0.1, 0.15, 0.2, and 0.3) were prepared by co-precipitation. Activity tests indicated that the Sm_0.15Mn_0.15Ti catalyst showed superior performances, with a NO conversion of 100% and N₂ selectivity above 87% at 180–300°C. The characterizations showed that Sm doping suppressed the crystallization of TiO₂ and Mn₂O₃ phases and increased the specific surface area and acidity. In particular, the surface area increased from 152.2 m²·g⁻¹ for Mn_0.3Ti to 241.7 m²·g⁻¹ for Sm_0.15Mn_0.15Ti. These effects contributed to the high catalytic activity. The X-ray photoelectron spectroscopy (XPS) results indicated that the relative atomic ratios of Sm³⁺/Sm and O_β/O of Sm_0.15Mn_0.15Ti were 76.77at% and 44.11at%, respectively. The presence of Sm contributed to an increase in surface-absorbed oxygen (O_β) and a decrease in Mn⁴⁺ surface concentration, which improved the catalytic activity. In the results of hydrogen temperature-programmed reduction (H₂-TPR), the presence of Sm induced a higher reduction temperature and lower H₂ consumption (0.3 mmol·g⁻¹) for the Sm_0.15Mn_0.15Ti catalyst compared to the Mn_0.3Ti catalyst. The decrease in Mn⁴⁺ weakened the redox property of the catalysts and increased the N₂ selectivity by suppressing N₂O formation from NH₃ oxidation and the nonselective catalytic reduction reaction. The in situ diffuse reflectance infrared Fourier transform spectra (DRIFTs) revealed that NH₃-SCR of NO over the Sm_0.15Mn_0.15Ti catalyst mainly followed the Eley–Rideal mechanism. Sm doping increased surface-absorbed oxygen and weakened the redox property to improve the NO conversion and N₂ selectivity of the Sm_0.15Mn_0.15Ti catalyst.