Background. Recent research suggests that an imbalance of the intestinal microbiota and a dysfunctional intestinal barrier might trigger irritable bowel syndrome (IBS). As probiotics have been reported to restore the intestinal microbiota and the gut barrier, the therapeutic potential of probiotics within IBS became of strong interest. Aim. To assess the efficacy of Bifidobacterium bifidum MIMBb75 in IBS. Methods. 122 patients were randomized to receive either placebo (N=62) or MIMBb75 (N=60) once a day for four weeks. The severity of IBS symptoms was recorded daily on a 7 point Likert scale. Results. MIMBb75 significantly reduced the global assessment of IBS symptoms by -0.88 points [95%CI: -1.07; -0.69] as compared to only -0.16 [95%CI: -0.32; 0.00] points in the placebo group (p<0.0001). MIMBb75 also significantly improved the IBS symptoms pain/discomfort, distension/bloating, urgency and digestive disorder. The evaluation of the SF12 sum scores showed a significant gain in quality of life within the bifidobacteria group. Furthermore, adequate relief was reported by 46.7% of the patients in the bifidobacteria and only by 11.3% of the patients in the placebo group (p<0.0001). Overall responder rates were 56.7% in the bifidobacteria group but only 21.0% in the placebo group (p=0.0001). MIMBb75 was well tolerated and adverse events were not different from placebo. Conclusion. B. bifidum MIMBb75 effectively alleviates global IBS and improves IBS symptoms simultaneously with an improvement of quality of life. Considering the high efficacy of MIMBb75 in IBS along with the good side effect profile, MIMBb75 is a promising candidate for IBS therapy.