Ulcerative colitis (UC) is a chronic inflammatory disease of the colon that is associated with dysbiosis in the gut microbiota. , a marine alga, is known for its anti-inflammatory, antioxidant, and gut microbiota-modulating properties. This study explored the mechanisms by which a 70% ethanol extract of may alleviate UC, through both in vitro and in vivo experiments. LC-MS/MS analysis revealed eckol, 7-phloroeckol, dieckol, phlorofucofuroeckol A, and fucofuroeckol as key phenolic compounds present in the extract. The administration of significantly improved symptoms in a dextran sulfate sodium (DSS)-induced colitis mouse model by reducing intestinal shortening, splenomegaly, and histological scores. Both cell and animal studies demonstrated that suppressed the release of inflammatory cytokines, downregulated the mRNA expression of genes related to the mTOR pathway, and reduced the p-mTOR/mTOR ratio. Microbiota analysis revealed that, while the Firmicutes/Bacteroidetes ratio was elevated in UC mice, administration normalized this imbalance, with a notable increase in the abundance of beneficial probiotics such as . In conclusion, a phenolic-rich extract of demonstrates significant potential as a dietary supplement to prevent and mitigate UC by modulating both the mTOR signaling pathway and gut microbiota composition.