Justice-involved youth (JIY) have high rates of behavioral health disorders, but few can access, much less complete, treatment in the community. Behavioral health treatment completion among JIY is poorly understood, even within treatment studies. Measurement, reporting, and rates of treatment completion vary across studies. This systematic review and meta-analysis synthesizes the literature on rates of treatment completion among JIY enrolled in research studies and identifies potential moderators. After systematically searching 6 electronic databases, data from 13 studies of 20 individual treatment groups were abstracted and coded. A meta-analysis examined individual prevalence estimates of treatment completion in research studies as well as moderator analyses. Prevalence effect sizes revealed high rates of treatment completion (pr = 82.6). However, analysis suggests a high likelihood that publication bias affected the results. Treatment groups that utilized family- or group-based treatment (pr = 87.8) were associated with higher rates of treatment completion compared to treatment groups utilizing individual treatment (pr = 61.1). Findings suggest that it is possible to achieve high rates of treatment completion for JIY, particularly within the context of family- and group-based interventions. However, these findings are limited by concerns about reporting of treatment completion and publication bias.