Background: Probiotic potential (efficacy and safety) of Bacillus coagulans SNZ 1969 has been studied in patients with constipation-predominant irritable bowel syndrome (IBS-C) and-diarrhea predominant IBS (IBS-D).Methods: This randomized, double-blind, two-arm, placebo-controlled parallel study randomized 92 patients (1:1) to receive either 500 million CFU of Bacillus coagulans SNZ 1969 (treatment group) or placebo (placebo group) twice daily for 60 days under two subtypes of IBS, IBS-D (n=46) and IBS-C (n=46). Primary outcomes were changes in IBS symptom severity noted using the gastrointestinal symptom rating scale-IBS version (GSRS-IBS) on days 30, 60, and 75, and the number of treatment responders defined by subject’s global assessment (SGA) of relief ≤3 and ≤2 at days 30 and 60, respectively. We also assessed patient’s quality of life.Results: The GSRS-IBS scores reduced from day 30 through 75 in both IBS groups treated with Bacillus coagulans SNZ1969 compared to placebo (p<0.05). Higher GSRS-IBS score was noted in patients with IBS-C in the treatment group (22.45±2.7) than the placebo group (3.55±3.02; p<0.0001), and this trend was similar in IBS-D patients (p<0.0001). Most patients (90%) with IBS-C and all with IBS-D responded to Bacillus coagulans SNZ 1969 compared to no responders with placebo (p<0.0001). The SF-8 scores significantly reduced in patients receiving Bacillus coagulans SNZ 1969 than placebo for both IBS subtypes. One adverse event unrelated to the study treatments was reported in IBS-D group.Conclusions: Bacillus coagulans SNZ 1969 is safe, effective in alleviating IBS-associated clinical symptoms, and improves quality of life.