Abstract Background There is growing interest in measuring the eudaimonic perspective of mental well-being (social and psychological well-being) alongside existing measures of the hedonic perspective of mental well-being (subjective well-being). The Flourishing Scale (FS) assesses core aspects of social-psychological functioning and is now widely used in research in practice. However, the reliability and validity of eudaimonic measures such as the FS has not yet been tested in people with low or moderate levels of well-being. This group is at risk for developing mental disorders and, therefore, an important target group for public mental health. Methods We extensively evaluated the psychometric properties of the 8-item FS in a sample of adults with low or moderate levels of well-being in The Netherlands (N = 275) using confirmatory factor analysis (CFA), item response theory analysis and a multitrait matrix. Results The unidimensional structure of the scale was confirmed with CFA and an adequate fit to the Rasch model. However, our sample showed positive skewness of the scale, but lacked measurement precision at the higher end of the social-psychological continuum. In general, the multitrait matrix demonstrated the convergent validity of the scale, with strong to weak correlations between the FS and (1) overall well-being, (2) social and psychological well-being (3) positive eudaimonic states, (4) hedonic states, (5) psychopathology and (6) personality traits. Nevertheless, relatively low correlations were found, specifically in comparison with the Mental Health Continuum-Short Form (MHC-SF). Conclusions The FS seems a reliable and valid instrument for measuring social-psychological functioning in adults with suboptimal well-being, but its use in intervention studies and clinical practice might be debatable. Therefore, the FS seems most suitable to include in epidemiological studies alongside existing hedonic measures to more fully capture mental well-being. Future research should examine the temporal stability of the FS and the consequences of the positive skewness and limited external validity of the scale found in the current study.