The employment of macroalgal biomass in anaerobic digestion (AD) systems could provide an alternative and renewable energy source and overcome the concern of algal influx. Sargassum fulvellum macroalgae was applied in the batch digesters as raw algal biomass (M_raw), enzymatically treated biomass (M_enz), reduced-sized biomass (M_red), chemically pretreated reduced-sized biomass with 0.36 mL g⁻¹ VS (M_acid1) and 0.18 mL g⁻¹ VS (M_acid2) of 2 M HCl and with 0.09 mL g⁻¹ VS (M_alkali1) and 0.04 mL g⁻¹ VS (M_alkali2) of 6 M NaOH. The results of this study indicated that the biologically treated digester (M_enz) enhanced the methane yield to 186.60 mL g⁻¹VS, which corresponds to 116.64% and 33.48% increases over those in the M_red and M_raw, respectively. Additionally, chemical pre-treatments of Sargassum fulvellum enhanced the methane outcome by 15.11%, 6.53%, 45.65%, and 37.01% for M_acid1, M_acid2, M_alkali1, and M_alkali2, respectively, compared with the M_red. This study emphasized that the utilization of raw untreated Sargassum fulvellum macroalgae through the thermophilic AD system was better than chemical and mechanical pre-treated feedstocks, which verified the eco-benefits and sustainable biomass management approach. The gas recovery of raw macroalgae can be further improved through the supplementation with a cellulase enzyme.