<p>Biochar amendment has been proposed to enhance thermophilic anaerobic digestion (AD), yet the influence of biochar physicochemical properties on co-digestion performance and microbial restructuring remains unknown. In this study, three biochars derived from bamboo, hog manure, and hickory shell were evaluated during thermophilic (55 °C) co-digestion of food waste and waste activated sludge. Compared with the control (310.5 ± 7.6 mL CH₄ /g volatile solids), biochar supplementation significantly increased cumulative methane yield, with hickory shell biochar achieving the highest production (393.4 ± 9.8 mL CH₄ /g volatile solids), corresponding to a 26.7% increase. Kinetic analysis showed that biochar increased the maximum methane production rate by up to 34.6% and shortened the lag phase by approximately one-third. Propionate accumulation was suppressed with hickory shell biochar, accompanied by enhanced soluble chemical oxygen demand removal and improved pH buffering. Biochar particle-size variation at the millimeter scale had a negligible effect. Microbial analysis revealed enrichment of hydrolytic <i>Clostridium</i>_sensu_stricto_1 and versatile methanogens such as <i>Methanosarcina</i> and <i>Methanoculleus</i> in biochar-amended systems. Functional prediction indicated an increase in carbohydrate metabolism, acetate conversion (<i>ackA, pta</i>), and methanogenesis genes (<i>mcrA/B/G</i>). Overall, biochar enhanced thermophilic AD through coordinated improvements in hydrolysis, syntrophic metabolism, and methanogenic activity.</p>