Abstract: Dehalogenative deuteration represents a strategically important synthetic methodology for deuterium labeling, characterized by its precise positional selectivity and high isotopic incorporation efficiency. In this study, we engineered a series of mesoporous-carbon-supported palladium catalysts (Pd@MC) to investigate the effects of Pd nanoparticle size and mesopore dimensions on dehalogenative deuteration performance. Comprehensive characterization of pore architecture and Pd particle size distribution was achieved through N₂ physisorption analysis and transmission electron microscopy. Kinetic studies of dehalogenative hydrogenolysis revealed that the bimodal mesoporous carbon-supported Pd catalyst (Pd@BMC) exhibited superior performance compared with the single-modal mesoporous structure of the small-pore mesoporous carbon-supported Pd catalyst (Pd@SMC) and the large-pore mesoporous carbon-supported Pd catalyst (Pd@LMC) for faster intraparticle diffusion kinetics, improving substrate accessibility to active sites. Moreover, small-sized Pd clusters demonstrate enhanced catalytic activity in H₂/D₂ dissociation, thereby improving the kinetics of dehalogenative hydrogenation/deuteration reactions. Consequently, the Pd@BMC catalyst enables rapid and precise dehalogenative deuteration of 4-bromoaniline at room temperature, achieving both deuteration efficiency and isolated yields exceeding 99%. The deuteration protocol demonstrates functional group tolerance for both electron-donating and electron-withdrawing substituents. Scale-up experiments and cycling stability tests further demonstrate the potential of Pd@BMC for deuterium labeling applications.