This study set out to re-examine the effectiveness of study abroad programs in second language (L2) acquisition through a multi-level meta-analysis. Overall, 42 primary studies published between 1995 and 2019 were identified, and in total 283 effect sizes were meta-analysed. This study implemented a three-level random effects model to account for the clustered, mutually dependent effect sizes often nested in the primary studies of L2 study abroad research. The results indicated a medium-to-large effect (g = 0.87) on study abroad language programs. Essentially, the featured moderators in general explained more heterogeneity variances at level 3 (i.e. the between-study level) than at level 2 (i.e. the within study level). For study abroad language learners, language acquisition is optimal when learners, in particular those of a lower proficiency level, take both formal and content-based language courses while living with host families. Learners’ age and pre-program training may not moderate the effectiveness of study abroad language programs. Importantly, this study further established that the length of study abroad programs are positively associated with learners’ language gains, but that an extended and prolonged domestic program does not necessarily lead to such gains. Research and pedagogical implications are further discussed based on the research findings.