This is a double-blind, randomized (group assignment by chance), placebo-controlled, multicenter trial which will be conducted over a 13.5-months period The main objective of the research is to demonstrate the efficacy of Saccharomyces boulardii CNCM I-745 on global IBS symptoms, measured by the Irritable Bowel Syndrome Symptom Severity Scale (IBS-SSS), in comparison to placebo after 8 weeks.

Secondary efficacy objectives are to evaluate the impact of Saccharomyces boulardii CNCM I-745 on quality of life of IBS patients and on global and individual IBS symptoms, using the IBS-SSS, in comparison to placebo at regular intervals over a 3-month treatment period. This research also aims to evaluate the proportion of patients who improved ≥ 50 points on IBS-SSS and the proportion of responders according to the European Medicines Agency (EMA) definition (based on the patient's global assessment of efficacy and on abdominal pain score), after 8 weeks of treatment. Secondary safety objective is to evaluate the safety profile and tolerability of Saccharomyces boulardii CNCM I-745 capsules 500 mg/day in IBS patients in comparison to placebo

Inclusion Criteria:

1. Male or female aged ≥ 18 and ≤ 65 years.
2. Diagnosis of IBS of any subtype, except constipation predominant (IBS-C), according to Rome IV criteria.
3. IBS-SSS total score ≥ 175 at inclusion.
4. Able and willing to maintain their nutrition habits throughout the study participation.
5. Able to understand and willing to comply with study requirements and to provide written informed consent.
6. For women of childbearing potential: willing to use one or more acceptable birth control method throughout the study participation.

Exclusion Criteria:

1. Diagnosis of IBS-C according to Rome IV criteria.
2. Patient with more than 5 bowel movements per day on average during the screening period, according to the patient's diary (BSFS).
3. Severe illness(es) or medical condition(s), including gastrointestinal pathologies (other than IBS): gastrointestinal ulcers, coeliac disease, inflammatory bowel disease, bowel cancer, acute or chronic diarrhea secondary to confirmed infectious gastroenteritis, or enteral or parenteral nutrition.
4. History of abdominal surgery (except for appendectomy, cholecystectomy, surgery for hemorrhoids or cesarian section, more than 6 months prior to inclusion).
5. Familial colorectal cancer syndrome (Lynch, Familial Adenomatous Polyposis).
6. Fecal transplant within 6 months prior to screening.
7. Use of products marketed as prebiotics, probiotics or synbiotics within 2 weeks prior to screening. These products, with the exception of the investigational product, will not be allowed during the trial. Regular cheese or yogurt containing lactic acid bacteria are not an exclusion criterion.
8. Systemic antibiotic or antimycotic treatment within 2 weeks prior to randomization. These treatments are not allowed during the study.
9. Laxatives, antibloating agents, antidiarrheal medication, antispasmodics, within 2 weeks prior to screening. These treatments are not allowed during the study, except loperamide which can be used as rescue medication.
10. Daily or regular non-steroidal anti-inflammatory drugs (NSAIDS) at doses above cardiovascular prophylaxis (low dose aspirin) are not allowed within 2 weeks prior to screening and throughout the study participation.
11. Use of opioids or narcotic analgesics, including tramadol and codeine, within 6 weeks prior to screening. These treatments are not allowed during the study.
12. Treatment with two or more antidepressant/anxiolytic/antipsychotic within 3 months prior to study entry or during the trial. Treatment with a single antidepressant or anxiolytic or antipsychotic agent before and during the trial is allowed provided that the dose is stable within 3 months prior to study entry and during the trial participation.
13. Treatment with anticholinergics for overactive bladder such as solifenacin, darifenacin, oxybutynin, tolterodine, fesoterodin, propiverin, trospium chloride, or mirabegron, within 1 week prior to screening. These treatments are not allowed during the study.
14. Allergy to yeast, especially Saccharomyces boulardii, or known hypersensitivity to one of the components.
15. Patients having a central venous catheter, critically ill patients, and immunocompromised patients.
16. Patients with rare hereditary problems of galactose or fructose intolerance, total lactase deficiency, glucose-galactose malabsorption or sucrase-isomaltase insufficiency.
17. Excessive alcohol consumption (>7 units/week) and/or drug abuse.
18. Other medical conditions or comorbidities, treatment, which in the opinion of the investigator, would interfere with study compliance or data interpretation.
19. Presenting any significant biological or clinical anomalies that are not compatible with participation in the study according to the investigator.
20. Participant at risk of pregnancy, pregnant or breastfeeding female.
21. Participant under guardianship or curatorship.
22. Participant under the protection of the Court or deprived of liberty.
23. Participant participating in another interventional clinical trial which could interfere with the trial's results or impact the other trial's results; or within 5 half-lives of the study investigational treatment, whichever is longer.
24. Participant whose current state of health does not allow him/her to give consent.