Sargassum horneri (S. horneri), a brown seaweed excessively proliferating along Asian coastlines, are damaging marine ecosystems. Thus, this study aimed to enhance nutritional value of S. horneri through lactic acid bacteria fermentation to increase S. horneri utilization as a functional food supplement, and consequently resolve coastal S. horneri accumulation. S. horneri supplemented fermentation was most effective with Lactiplantibacillus pentosus SH803, thus this product (F-SHWE) was used for further in vitro studies. F-SHWE normalized expressions of oxidative stress related genes NF-κB, p53, BAX, cytochrome C, caspase 9, and caspase 3, while non-fermented S. horneri (SHWE) did not, in a H2O2-induced HT-29 cell model. Moreover, in an LPS-induced HT-29 cell model, F-SHWE repaired expressions of inflammation marker genes ZO1, IL1β, IFNγ more effectively than SHWE. For further functional assessment, F-SHWE was also treated in 3T3-L1 adipocytes. As a result, F-SHWE decreased lipid accumulation, along with gene expression of adipogenesis markers PPARγ, C/EBPα, C/EBPβ, aP2, and Lpl; lipogenesis markers Lep, Akt, SREBP1, Acc, Fas; inflammation markers IFN-γ and NF-κB. Notably, gene expression of C/EBPβ, IFN-γ and NF-κB were suppressed only by F-SHWE, suggesting the enhancing effect of fermentation on obesity-related properties. Compositional analysis attributed the protective effects of F-SHWE to acetate, an organic acid significantly higher in F-SHWE than SHWE. Therefore, F-SHWE is a novel potential anti-obesity agent, providing a strategy to reduce excess S. horneri populations along marine ecosystems.