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Background The role of microbiota-targeted therapies in ulcerative colitis (UC) has gained significant attention due to their capacity to restore gut homeostasis, thereby optimizing disease management. This trial aims to assess the efficacy of a multi-strain probiotic as an adjunct therapy to the standard of care for patients with mild UC.Methods Mild UC patients were randomly assigned to receive either a multi-strain probiotic (Group G, comprising of three Lactobacillus and three Bifidobacterium strains) or a placebo (Group J) twice daily for 12 weeks, alongside their standardized treatment with mesalazine. Stool assessment for 16S rRNA sequencing, faecal calprotectin (FCAL), serum C-reactive protein (CRP), erythrocyte sedimentation rate (ESR), white cell count and clinical assessment using partial Mayo score (pMS) were measured pre and post treatment.Results A total of 59 patients (Group G, n=30; Group J, n=29) had completed the trial. At post-intervention, linear discriminant analysis effect size (LEfSe) revealed distinct microbial associations in both groups. Group G was differentially abundant with Eubacterium, Negativibacillus, and Paraprevotella clara, whereas Group J was enriched with Copromonas, Enterocloster, Peptoniphilus, and Parasutterella excrementihominis. Clinically, Group G demonstrated a significant reduction in pMS (per-protocol (PP) and intention to treat (ITT) analyses, P=0.0003), along with reduced CRP level (PP, P=0.0269). Conversely, Group J also showed pMS improvement (PP, P=0.0273), without any corresponding improvement in any inflammatory markers. Moreover, post-treatment FCAL levels differed significantly between groups (ITT, P=0.0163), with Group G maintaining normalized levels (median: 15.1mg/kg), while Group J exhibited elevated levels (median: 51.5mg/kg). The significant abundance of commensal taxa in Group G was associated with sustained clinical improvement, facilitating both symptom relief and reduction of inflammatory response. In contrast, the enrichment of pathogenic taxa specifically P. excrementihominis, a microbial marker associated with UC and colitis-associated colorectal cancer, correlated with persistent intestinal inflammation despite symptomatic relief.Conclusions A multi-strain probiotic formulation demonstrated promising efficacy as an adjunctive therapy to standard of care treatments in patients with mild ulcerative colitis.