Catalytic activity and hydrothermal stability are both crucial for the application of the selective catalytic reduction of NO_x with NH₃ (NH₃-SCR) catalyst in diesel vehicles. In this study, a tin (Sn)-modified Ce–Nb mixed-oxide catalyst was synthesized as an NH₃-SCR catalyst for NO_x emission control. After the introduction of Sn, both the NH₃-SCR activity and the hydrothermal stability of the catalyst were remarkably promoted. Even after hydrothermal aging at 1000 °C, the developed Ce₁Sn₂Nb₁O_x catalyst achieved more than 90% NO_x conversion at 325–500 °C. Various methods, including N₂-physisorption, X-ray diffraction, in-situ high-temperature X-ray diffraction, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray absorption fine-structure spectroscopy, temperature-programmed reduction of hydrogen, temperature-programmed desorption of ammonia, and density functional theory calculations were used to investigate the promotional effects induced by the Sn species. The characterization results showed that the addition of Sn not only promoted the formation of the Ce–Nb active phase but also improved its thermal stability, contributing to the excellent NH₃-SCR performance and hydrothermal stability. This study provides an excellent sintering-resistance catalyst for the application of diesel engine NO_x emission control.