Purpose: The Comprehensive assessment of Acceptance and Commitment Therapy (CompACT) is a 23-item questionnaire measuring psychological flexibility, a quality of life protective factor. An 18-item version was recently produced. We assessed validity and reliability of CompACT, and equivalence of paper and electronic (eCompACT) versions in people with multiple sclerosis (PwMS) in Italy, Germany and Spain. Methods: We used confirmatory factor analysis and assessed CompACT-23 and CompACT-18 measurement invariance between the three language versions. We assessed construct validity (Spearman's correlations) and internal consistency (Cronbach’s alpha). Test–retest reliability (intraclass correlation coefficient, ICC) and equivalence of paper and eCompACT (ICC and linear regression model for repeated measures) were assessed in subsamples of PwMS. Results: A total of 725 PwMS completed the study. The three-factor structure of the CompACT-23 showed poor fit (RMSEA 0.07; CFI 0.82; SRMR 0.08), while the fit of the CompACT-18 was good (RMSEA 0.05; CFI 0.93; SRMR 0.05). Configural and partial metric invariance were confirmed, as well as partial scalar invariance (reached when five items were allowed to vary freely). The CompACT-18 showed good internal consistency (all alpha ≥ 0.78); and test–retest reliability (all ICCs ≥ 0.86). Equivalence between paper and eCompACT was excellent (all ICCs ≥ 0.86), with no mode, order, or interaction effects. Conclusion: Results support using the refined CompACT-18 as a three-factor measure of psychological flexibility in PwMS. Paper and eCompACT-18 versions are equivalent. CompACT-18 can be used cross-culturally, but sub-optimal scalar invariance suggests that direct comparison between the three language versions should be interpreted with caution.