Sandplay therapy is a cross-cultural, psychodynamic, nondirective, multisensory psychotherapy method founded by Dora Kalff. Sandplay is used with children and adults with a range of mental health problems. Despite sandplay’s growing popularity, its empirical evidence base is less developed than more well-known therapies. This international study provides a meta-analysis of the available quantitative outcome studies in order to summarize the growing evidence base of sandplay. The meta-analysis specifically examined emotional and behavioral outcome measures of treatment with sandplay therapy. The initial search identified 1,715 potential records from over 16 countries. After screening, 40 studies from eight countries representing 1,284 participants met the inclusion criteria. Mean effect sizes were calculated using a random effects model with the Comprehensive Meta-Analysis (CMA) program. The overall effect size was large (Hedges’ g = 1.10). Large effect sizes were maintained for internalizing, externalizing, and attention-deficit/hyperactivity disorder (ADHD) symptoms. Improved effect sizes were associated with individual treatment over the group format. These results suggest that sandplay therapy is an effective treatment method for children and adults with a wide variety of mental health concerns. Limitations and suggestions for further research are discussed.