AbstractBrown algae are gaining traction as biorefinery feedstocks due to their advantages such as rapid growth and carbon dioxide sequestration. Saccharina japonica has high potential due to its high carbohydrate content, especially mannitol (26.7%). In this study, a biorefinery process for S. japonica was designed, with focusing on sugar conversion and bioconversion into lactic acid, a valuable platform chemical utilized in various industries. The existing sugar conversion process of S. japonica has been investigated by focusing on enzymatic or acid‐catalyzed hydrolysis, but not hot water extraction although mannitol can be easily recovered using water. The effect of temperature (60–120°C) on the mannitol yield from S. japonica was investigated, and a mannitol yield of 208 g/kg biomass was achieved at the optimal temperature of 100°C (about 78% of the theoretical maximum yield). This study emphasizes that this simple process has considerable potential for application as over 80% of the fermentable carbohydrates in S. japonica were mannitol. Then, S. japonica extract was applied to lactic acid production. First, lactic acid production of four bacterial strains was tested in a mannitol medium, and Lacticaseibacillus rhamnosus was selected as the superior producer, showing 1.93 to 2.92 times better lactic acid titer than others. Next, the optimal feeding concentration of mannitol was determined to be 20 g/L, which was all consumed by L. rhamnosus. Finally, S. japonica extract was applied to lactic acid production by L. rhamnosus, and the results showed similar fermentation profiles with the control medium: lactic acid production, 18.81 g/L (control: 18.97 g/L); lactic acid conversion, 95.1% (control: 95.9%); cell growth (OD600 nm), 8.9 (control: 7.4). The lactic acid yield in the designed biorefinery process was estimated to be 195.6 g/kg biomass, thus S. japonica has high potential as a biorefinery feedstock to produce valuable bioproducts, including lactic acid.