The belief that happiness is fragile—that it is fleeting and may easily turn into less favourable states—is common across individuals and cultures. However, not much is known about this belief domain and its structure and correlates. In the present study, we use multigroup confirmatory factor analysis and multilevel modelling to investigate the measurement invariance, cross-level isomorphism, predictive validity, and nomological network of the fragility of happiness scale across 15 nations. The results show that this scale has good statistical properties at both individual and cultural levels, and is associated with relevant psycho-social concepts in expected directions. The importance of the results, limitations, and potential directions for future research are discussed.