The pharmacological values of marine algal polysaccharides on gut health are being recognized in recent research. However, the protective effect of degraded polysaccharides from (PHP-D) on the colonic mucosal barrier damaged in ulcerative colitis is poorly understood. The purpose of this study was to investigate how PHP-D could maintain the integrity of colonic mucosal layer mediated by microbiota in a dextran sulfate sodium (DSS)-induced colitis mouse model. Structural analysis revealed that PHP-D had a typical porphyran structure having a backbone of alternating (1 → 3)-linked β-d-galactopyranose units linked to either (1 → 4)-3,6-anhydro-α-l-galactopyranose units or (1 → 4)-linked α-l-galactose-6-sulfate units. An study demonstrated that PHP-D treatment reduced the severity of DSS-induced ulcerative colitis. 16S rRNA phylogenetic sequencing revealed that PHP-D affected the diversity of gut microbiota with an increase of , , and species. Similarly, PHP-D increased levels of short-chain fatty acids. Furthermore, PHP-D restored mucus thickness and improved the expression of tight junction proteins. This work demonstrates that PHP-D is capable of enhancing a colonic mucosal barrier. These outcomes offer unique perspectives on the potential application of as a promising natural product for the management of ulcerative colitis.