Functional constipation affects 10–12% of the global population, significantly impairing quality of life. Current treatments are often insufficient, creating a need for safe, effective long-term strategies. Probiotics are promising, particularly spore-forming Bacillus coagulans. While the B. coagulans MTCC 5856 (LactoSpore®) strain has shown efficacy in diarrhea-predominant irritable bowel syndrome, its effectiveness in managing functional constipation is clinically unverified.The primary objective of this study was to evaluate the efficacy and safety of B. coagulans MTCC 5856 supplementation on clinical symptoms and the fecal microbiome in adults with functional constipation.This 8-week, randomized, double-blind, placebo-controlled trial enrolled 80 adults (aged 19–75) with functional constipation, as defined by the Rome IV criteria. Participants were randomly assigned (1:1) to receive B. coagulans MTCC 5856 (2 × 10⁹ CFU/day) or placebo. The primary endpoint was the change from baseline in the constipation Visual Analogue Scale (VAS) score at weeks 4 and 8. Secondary endpoints included IBS Quality of Life (IBS-QOL), IBS Symptom Severity Score (IBS-SSS), and gut microbiome composition. An intention-to-treat (ITT) analysis was performed using linear mixed-effects models.In the ITT analysis (n=80), the B. coagulans group showed significantly greater improvement in constipation VAS scores versus placebo at week 8 (adjusted mean difference: 28.8; 95% CI, 14.1–43.4; p < 0.001) and week 4 (p = 0.012). At week 8, significant improvements were also seen in VAS-IBS (p < 0.001), IBS-QOL (p = 0.033), and IBS-SSS (p = 0.019). However, no significant differences in fecal microbiome composition or diversity were found between the groups. No intervention-related adverse events were reported.This study is the first RDBPC to demonstrate that B. coagulans MTCC 5856 significantly improves symptoms in adults with functional constipation. The fact that this strain also shows benefit in IBS-D suggests a potential bidirectional regulatory effect on gut motility. Symptomatic improvement occurred without significant alterations to the overall microbial composition, suggesting that the mechanism of action may be functional (e.g., metabolic) rather than structural.Eight-week supplementation with B. coagulans MTCC 5856 was effective in improving clinical symptoms and quality of life in adults with functional constipation and demonstrated an excellent safety profile.