Ulcerative colitis (UC) is associated with gut microbiota imbalance, and probiotics may restore gut flora and improve intestinal health. Sea buckthorn, which is rich in antioxidants and anti-inflammatory compounds, may enhance these effects when combined with probiotics. In this study, we incorporated our screened strain Lactiplantibacillus plantarum YHG-87 (L. plantarum YHG-87) into sea buckthorn fermented milk to examine its additional benefits in a dextran sulfate sodium (DSS)-induced UC mouse model. Five groups (n = 6) were included: healthy control, DSS-induced colitis, regular fermented milk, sea buckthorn fermented milk, and sea buckthorn probiotic fermented milk (SC group). Results indicated that SC group significantly alleviated UC symptoms, such as weight loss, bloody stools, and colon shortening (p < 0.05), and these improvements were more pronounced than those observed in the sea buckthorn fermented milk group. Moreover, the SC group exhibited stronger anti-inflammatory and antioxidant effects, including reduced IL-6, increased IL-4 and IL-10, elevated glutathione, and reduced myeloperoxidase and malondialdehyde (p < 0.05). Additionally, the SC intervention led to notable shifts in gut microbial community composition. In conclusion, the addition of L. plantarum YHG-87 to sea buckthorn fermented milk provided enhanced protective effects against UC, suggesting that the combination of bioactive plant components with selected probiotics offers promising potential for UC prevention and therapy.