In this research, we conducted two studies to validate a multidimensional scale of fan engagement behavior. In Study 1, we generated survey items through a systematic review of the relevant literature, collected data from fans of professional baseball (n = 319) and soccer (n = 301), and provided evidence for the construct and concurrent validity of the scale composed of six dimensions. In Study 2, we reassessed construct validity in professional baseball (n = 582) and found that fan engagement behavior was represented by the proposed six dimensions with a final list of 21 items. Further, our predictive analysis throughout a season showed that fan engagement behavior fully mediated the relationship between predictor (team identification and awareness of fan engagement initiatives) and outcome variables (media viewing frequency, attendance frequency, and flourishing). The developed scale advances our understanding of fans’ voluntary actions that are culturally embedded in spectator sport.