This study investigated the potentiality of methane production from alginate-extracted (AEWLJ) and non-extracted (NAEWLJ) waste of Laminaria japonica through batch anaerobic fermentation in mono- and co-digestion with rice straw (RS) at different mixing ratios. Optimal C/N ratio was demonstrated, and system stability was monitored in terms of the total ammonia nitrogen, total volatile fatty acids, and pH throughout the digestion period. The results show that the combination of AEWLJ/RS at 67% mixing ratio generated the highest biogas yield of 247 NmL/gVS, which was 36% higher than the AEWLJ alone. The synergetic effect was clearly observed leading to an increase in the total methane yield up to 78% and 88%, respectively, for arrays of AEWLJ/RS and NAEWLJ/RS. The kinetic model showed a high coefficient of determination (R2 ≥ 0.9803) when the modified Gompertz model was applied to predict methane production. These outcomes support the possibility of an integrated biorefinery approach to attain value-added products in order to achieve circular economies.