Ceramic-supported Pt catalysts are promising for durable polymer electrolyte fuel cells, where conventional carbon support corrosion remains a major limitation. In this study, SnO₂-supported Pt catalysts were systematically investigated using hard X-ray photoelectron spectroscopy (HAXPES), X-ray absorption spectroscopy (XAS/EXAFS), and synchrotron infrared (IR) spectroscopy to clarify the interfacial electronic state, local structure, and surface functional groups. The spectroscopic results provide direct evidence for electron donation from Pt to SnO₂ and PtSn alloy formation at the catalyst–support interface, with these electronic modifications becoming more pronounced at higher Pt loadings. Synchrotron IR measurements did not clearly resolve hydroxyl or other surface functional groups on Pt/Nb-SnO₂ under the current conditions, indicating that their direct contribution to catalytic behavior is likely limited, although weak signals cannot be excluded. These findings elucidate the interfacial evolution of Pt/Nb-SnO₂ systems and highlight the critical role of metal–support interactions in tuning the properties of ceramic-supported Pt catalysts, providing essential design guidelines for developing stable, carbon-free electrocatalysts for next-generation fuel cells.