Increased attention has been paid in recent years to the study of human flourishing, yet few established tools exist to measure flourishing in marginalized youth populations. The validity of the adolescent version of the Flourishing Index (AFI) was recently substantiated in a sample of youth aging out of foster care. AFI scores are interpreted here as a brief, multidimensional indicator of flourishing among U.S. youth ages 18 to 24 with foster care experience. Building on prior evidence based on internal structure and measurement invariance, this study examines additional validity evidence based on relations with other variables. Convergent and discriminant patterns of association with the PERMA-Profiler, a widely used multidimensional well-being framework, are examined. Incremental prediction of general self-efficacy beyond PERMA was examined using hierarchical regression. These findings provide additional validity evidence for the use of AFI scores in this population. Further evidence is needed before AFI scores are used for individual-level decisions or for program accountability.