This study aimed to perform a cross-country validation of the Arabic version of the World Health Organization 5-item (WHO-5) Well-Being Index, in terms of factor structure, composite reliability, cross-gender measurement invariance and concurrent validity. We carried out a cross-sectional, web-based study on a total of 3,247 young adults (aged 18-35 years) from six Arab countries (Tunisia, Lebanon, Egypt, Jordan, Morocco and Kuwait). Confirmatory Factor Analysis showed that the one-factor model demonstrated acceptable fit across all six countries. In addition, the Arabic WHO-5 Well-Being Index yielded high reliability coefficients in samples from each country (McDonald's and Cronbach's = .92-.96), across genders ( = .95 in men and .94 in women) and age groups ( = .94/α = .94 in participants aged ≤25 years and =.96/ =.96 in those aged ≥26 years). Multi-group analyses demonstrated that configural, metric and scalar invariance were supported across gender, countries and age groups. Regarding concurrent validity, WHO-5 Well-being scores were strongly and significantly inversely correlated with depression, anxiety, stress, suicidal ideation and insomnia severity. This study provides a brief, valid and reliable Arabic version of the WHO-5 Well-Being Index that can be applied cross-nationally among Arabic-speaking young adult populations for screening and research purposes.