Utilization of macroalgal biomass through the anaerobic digestion (AD) system can overcome algal pollution while providing alternative and renewable energy as fuel scarcity increases. To achieve this objective, Sargassum fulvellum biomass was used as the feedstock for the batch AD process. Original-sized algal biomass (So) of 106 µm–4.75 mm particle size, reduced-sized biomass of 75–850 µm (mechanically pretreated, Sr), chemically pretreated reduced-sized wet biomass with 40 mL/L (Sac1), 20 mL/L (Sac2) of 2 M HCl and with 10 mL/L (Sal1) and 5 mL/L (Sal2) of 6 N NaOH, and biologically pretreated original particle size biomass (Se) with 1 mL/L of cellulase enzyme were employed as feedstocks for digestion. Mechanical pretreatment of Sargassum fulvellum (Sr) without chemical addition resulted in 142.91 ± 0.004 mL CH₄/gVS, which is higher than chemically pretreated reduced-sized macroalgae in So, Sac1, Sac2, Sal1, and Sal2 by 52.34%, 9.83%, 15.89%, 12.73%, and 18.26%, respectively. Biological treatment reduced methane yield in Se by 9.49% than the original-sized algal biomass (So). In addition, the rate of hydrolysis and maximum biomethane production potential improved after mechanical pretreatment by a maximum of 45.60% and 48.71%, respectively. This study indicates that the utilization of marine biomass as an alternative resource for biomethane production can be achieved, with an optimum methane production from mechanically pretreated macroalgae without chemical addition.