Ulcerative colitis (UC) is a multifactorial disease characterized by aberrant mucosal immune activation in response to intestinal dysbiosis. Contemporary management strategies aim to target inflammation and microbiome alterations while reducing relapse risk. A multidimensional assessment integrating clinical, inflammatory, immune, and microbial endpoints may better capture therapeutic effects beyond symptom control. This video aims to evaluate whether supplementation with Lactobacillus rhamnosus GG co-formulated with vitamin D3 (Dicoflor IBD Immuno) as an adjunct to optimized mesalamine (5-ASA) is associated with coordinated changes across clinical and biological domains in mild-to-moderate UC, using a multidimensional assessment framework. This single-center, randomized, double-blind, placebo-controlled pilot trial was conducted at Fondazione Ca' Granda IRCCS Policlinico di Milano between May 2022 and May 2024. Thirty-six patients with mild-to-moderate UC receiving optimized 5-ASA were randomized to LGG+VitD3 (ALD3) or placebo (AP) for 4 weeks. Clinical activity, health-related quality of life (HRQoL), fecal calprotectin, peripheral immune cell subsets, and gut microbiota composition were assessed at baseline and week 4. Both 5-ASA-LGG+VitD3 (ALD3)- and 5-ASA-placebo (AP)-treated patients showed significant improvement in clinical activity and HRQoL, without between-group differences. A higher proportion of clinical responders was observed in the ALD3 group, although this was not statistically significant. LGG+VitD3-supplemented patients showed reduced fecal calprotectin levels and increased frequencies of IL-22-producing CD4+ T cells. Microbiome analysis revealed enrichment of short-chain fatty acid-producing taxa, including Coprococcus and Fusicatenibacter, in ALD3-treated patients. This video concludes that in patients with mild UC receiving optimized 5-ASA, LGG+VitD3 supplementation does not improve short-term clinical outcomes beyond placebo but is associated with favorable modulation of inflammatory, immune, and microbial parameters, supporting the relevance of multidimensional biological endpoints in adjunctive UC management.