Whilst elevated reaction temperature in thermal-assisted photocatalysis is widely known to enhance reaction kinetics, its influence on product selectivity remains underexplored, especially lacking mechanistic insights into the role of thermal regulation on reaction pathways. Herein, we developed a temperature-controllable photocatalytic platform to unravel the thermal modulation mechanism for reaction selectivity, utilizing the photocatalytic ammonia oxidation reaction over TiO₂-supported Pt nanoparticles as a model. Under ultraviolet (UV)–visible light irradiation at 80 °C, this platform exhibited outstanding ammonia oxidation activity (0.78 mmol·g⁻¹·h⁻¹) and NO₃⁻ selectivity (87.0%), accompanied by a remarkable apparent quantum efficiency of 64.8% at 365 nm for hydrogen evolution over TiO₂-supported Pt nanoparticles. Further, a series of temperature-dependent characterizations, including photo-electrochemical testing, transient surface photovoltage spectra, and in situ electron paramagnetic resonance analyses revealed that the elevated temperature enhanced carrier separation efficiency and facilitated ·OH radical generation. This consequently boosted the complete oxidation of ammonia into nitrate while suppressing nitrite generation. This work elucidates the mechanism of thermal-assisted photocatalysis in modulating ·OH concentration and carrier dynamics, paving the way for designing efficient ·OH-mediated photocatalytic systems.