Vanadia-based catalysts have been widely used for catalyzing various reactions, including their long-standing application in the deNOx process. It has been commonly considered that various vanadium species dispersed on supports with a large surface area act as the catalytically active sites. However, the role of crystalline V₂O₅ in selective catalytic reduction of NOx with NH₃ (NH₃-SCR) remains unclear. In this study, a catalyst with low vanadia loading was synthesized, in which crystalline V₂O₅ was deposited on a TiO₂ support that had been pretreated at a high temperature. Surprisingly, the catalyst, which had a large amount of crystalline V₂O₅, showed excellent low-temperature NH₃-SCR activity. For the first time, crystalline V₂O₅ on low-vanadium-loading catalysts was found to be transformed to polymeric vanadyl species by the adsorption of NH₃ . The generated active polymeric vanadyl species played a crucial role in NH₃-SCR, leading to remarkably enhanced catalytic performance at low temperatures. This new finding provides a fundamental understanding of the metal oxide-catalyzed chemical reaction and has important implications for the development and commercial applications of NH₃-SCR catalysts.