Ammonia (NH 3 ) slip from diesel vehicle aftertreatment systems and internal combustion engines fueled by NH 3 or NH 3 /H 2 poses serious environmental problems. Ag-based catalysts are widely used for the selective catalytic oxidation of NH 3 to N 2 (NH 3 -SCO), and their performance is greatly dependent on the state of Ag, which is influenced by the anchoring sites on the support. Despite efforts to identify the direct anchoring sites of metal atoms on TiO 2 , conflicting views persist. Here, we compared the correlation between Ag dispersion and the content of hydroxyl (OH) groups or defects on TiO 2 and conducted density functional theory (DFT) calculations, and the results confirmed that the surface OH groups of TiO 2 serve as the direct anchoring sites for Ag. By modulating the OH group content through thermal induction, the optimal OH group content on TiO 2 -800 resulted in more metallic Ag nanoparticles (Ag 0 NPs) in larger sizes, leading to the development of an excellent NH 3 -SCO catalyst. Moreover, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), kinetic studies, and DFT calculations suggested that more Ag 0 NPs in larger sizes on 10Ag/TiO 2 -800 were conducive to O 2 activation and NH 3 dissociation. Our findings provide new insights for designing efficient NH 3 -SCO catalysts, and OH groups as direct anchoring sites could be extended to other metals and supports for the rational design of catalysts.