Selective catalytic oxidation (SCO) of NH3 to harmless N2 and H2O is an ideal technology for its removal. To develop air purification systems for a living environment, catalysts that can work at room temperature with high selectivities to N2 are required. However, it has been a technical challenge because the reported catalysts either needed high operating temperatures or showed low selectivities to N2. In this study, we first demonstrated that acidic metal-oxide-supported gold catalysts showed good N2 selectivities compared with that of other metal-oxide-supported gold catalysts. A gold catalyst with niobium oxide synthesized by the hydrothermal method as a support showed high catalytic activity and high selectivity to N2 at low temperatures (18% NH3 conversion with 100% N2 selectivity at 25 °C) and at high temperatures (100% NH3 conversion with 95% N2 selectivity at 245 °C). Important roles of Brønsted acid sites and formation of active oxygen sites in improving N2 selectivity were revealed in this study. To the best of our knowledge, this is the first report of efficient catalysts that presented high NH3 conversion with high N2 selectivity at 25 °C which will offer great scopes for commercial applications related to control of odors. In addition, this breakthrough finding that acid sites would greatly affect N2 selectivity and catalytic activity will provide a new trend in designing efficient catalysts not only for SCO of NH3 but also for the other selective catalytic oxidation.