Positive mental health (PMH), defined as the presence of general emotional, psychological, and social well-being is an important factor of general mental health. To allow for valid comparisons of PMH across different groups of interest, measurement invariant instruments are needed. The present study tested the measurement invariance of the nine-item Positive Mental Health Scale (PMH-Scale) across eight countries (i.e., France, Germany, Poland, Russia, Spain, Sweden, the United Kingdom, and the United States) as well as across genders, and age groups. Population-based online-panel surveys were conducted (N per country > 1,000). The PMH-Scale showed good to excellent internal consistency as well as unidimensionality in all subsamples. Convergent validity was supported by positive correlations with the Perceived Social Support Questionnaire (F-SozU K-6), discriminant validity was shown be negative correlations with the subscales of the Depression Anxiety Stress Scales 21 (DASS-21). The PMH-Scale showed scalar measurement invariance across countries, genders, and age groups. Comparisons of latent means showed small to medium group differences, with highest levels of PMH found in the French and US-American samples as well in participants between 45 and 54 years of age. Lowest levels of PMH were found in Russian participants. Gender differences in PMH were negligible. The PMH-Scale can be used to meaningfully compare the levels of PMH across the eight investigated countries, women, and men, as well as younger and older individuals. (PsycInfo Database Record (c) 2022 APA, all rights reserved).