Ulcerative　colitis　(UC)　is　characterized　by　impaired　gut　barrier,　dysregulated　immune　responses　and　pronounced　gut　dysbiosis.　Euglena　gracilis　(EG),　rich　in　β-1,3-glucan　(EGP),　exhibits　immunomodulatory　properties,　yet　its　effects　on　colitis　and　EGP＇s　role　as　a　core　bioactive　component　are　unclear.　The　aim　of　this　study　was　to　investigate　the　protective　effects　of　EGP　against　UC　by　targeting　gut　barrier,　T-cell　immunity　and　gut　microbiota.　Results　indicated　that　EG　and　EGP　effectively　improved　the　body　weight,　colon　growth　and　reduced　disease　activity　index　of　the　DSS-induced　mice.　Both　treatments　also　significantly　suppressed　the　level　of　TNF-α　and　IL-6,　restored　gut　barrier　by　upregulating　ZO-1　and　balanced　Th17/Treg　cells　ratio.　Microbiota　analysis　revealed　EG　and　EGP　reshaped　gut　microbiota　composition,　with　an　increase　in　beneficial　strains,　particularly　within　the　Bacteroidota　phylum.　Metabolomics　linked　these　changes　to　enhanced　amino　acid　metabolism.　Bacteroides　fragilis,　a　Bacteroidota　member,　displayed　similar　anti-colitis　bioactivity.　In　vitro　fermentation　with　fecal　samples　from　UC　patients　confirmed　EGP＇s　role　in　reshaping　gut　microbiota,　increasing　beneficial　families　such　as　Clostridiaceae　and　Lactobacillaceae,　while　enhancing　tryptophan　metabolism　with　anti-inflammatory　indoles.　These　findings　identify　EGP　as　the　core　active　component　of　EG,　highlighting　its　potential　in　UC　prevention　through　microbiota　modulation,　gut　barrier　support　and　immune　regulation.　©　2025　Elsevier　B.V.