Background: Bowel symptoms are commonly experienced by patients with Multiple sclerosis (PwMS), but no specific questionnaire validated in this population allows a rigorous assessment. Objective: Validation of a multidimensional questionnaire assessing bowel disorders in PwMS. Methods: A prospective, multicenter study was conducted between April 2020 and April 2021. The STAR-Q (Symptoms’ assessmenT of AnoRectal dysfunction Questionnaire), was built in 3 steps. First, literature review and qualitative interviews were performed to create the first version, discussed with a panel of experts. Then, a pilot study assessed comprehension, acceptation and pertinence of items. Finally, the validation study was designed to measure content validity, internal consistency reliability (alpha coefficient of Cronbach) and test–retest reliability [intraclass correlation coefficient (ICC)]. The primary outcome was good psychometric properties with Cronbach's α > 0.7 and ICC > 0.7. Results: We included 231 PwMS. Comprehension, acceptation and pertinence were good. STAR-Q showed a very good internal consistency reliability (Cronbach's α = 0.84) and test-retest reliability (ICC = 0.89). Final version of STAR-Q was composed of 3 domains corresponding in symptoms (Q1–Q14), treatment and constraints (Q15–Q18) and impact on quality of life (Q19). Three categories of severity were determined (STAR-Q ≤ 16: minor, between 17 and 20: moderate, and ≥ 21: severe). Conclusions: STAR-Q presents very good psychometric properties and allows a multidimensional assessment of bowel disorders in PwMS. Level of evidence: 2