Background/Objectives: The worldwide occurrence of ulcerative colitis (UC) is increasing, but existing treatments frequently suffer from limited effectiveness and notable side effects. walnut green husk polysaccharide (WGHP) has been shown to exhibit anti-inflammatory and immunomodulatory activities; however, its specific potential and mechanisms of action against colitis remain unclear. This study aimed to evaluate the effectiveness of purified WGHP on (dextran sulfate sodium) DSS-induced UC and elucidate the underlying mechanisms. Methods: WGHP-2-2, a primary acidic polysaccharide fraction, was extracted from crude WGHP and analyzed through chromatography and spectroscopy. The therapeutic efficacy of WGHP-2 was assessed using a murine model of DSS-induced UC. Assessments included disease severity (DAI, colon length, histopathology), inflammatory markers (tissue IL-6, TNF-α, IL-10), and intestinal barrier integrity (Claudin-5, Occludin, ZO-1). Results: WGHP-2-2 is an acidic polysaccharide with a molecular weight of 15.29 kDa. Its composition includes glucosamine, rhamnose, glucuronic acid, galacturonic acid, glucose, galactose, and arabinose, with respective molar ratios of 0.55, 8.48, 3.06, 65.99, 4.49, 10.86, and 6.57. Methylation and NMR analyses revealed a backbone mainly composed of →4)-α-D-GalpA-(1→ and →2)-α-D-Rhap-(1→ linkages, with side chains or terminal residues such as T-Rhap, T-Galp, T-Glcp, and T-Araf. In vivo, WGHP-2 significantly mitigated DSS-induced UC symptoms in a dose-dependent manner. Specifically, the high-dose group (123 mg/kg) markedly attenuated colon shortening and improved histological architecture, including the restoration of colonic crypts. WGHP-2 effectively reduced pro-inflammatory cytokines IL-6 and TNF-α in colon tissues, while increasing the anti-inflammatory cytokine IL-10. Conclusions: WGHP-2 mitigates DSS-induced UC by inhibiting pro-inflammatory cytokines (IL-6, TNF-α), increasing IL-10 levels, and improving intestinal barrier integrity through the upregulation of tight junction proteins. These results position WGHP-2 as a promising lead compound for developing functional foods for UC.