Abundant research has indicated fluency features as meaningful predictors of second language proficiency. However, the extent to which different fluency dimensions and features can predict proficiency remains underexplored. This meta-analysis employed a multilevel modeling approach to synthesize fluency-proficiency relationships from 71 empirical studies from 1959-2023. Additionally, we examined several moderator variables, including task type, learning context, age, and proficiency measure. The correlations found were strongly positive for "speed" (r = 0.55), moderately positive for "productivity" (r = 0.38), moderately negative for "breakdown" (r = -0.33), and weakly negative for "repair" (r = -0.11). Moderator analyses revealed that task, learning context, and proficiency measure influence fluency-proficiency relationships in the repair, productivity, and speed dimensions, respectively. Post hoc analyses also suggested that the operationalization of breakdown features might make a difference in fluency--proficiency relationships. This study has both theoretical and methodological implications for second language fluency research.