Evaluation of evidence-based treatments is important for adolescents and adults with autism spectrum disorders (ASD) given the increasing number of interventions available and the prevalence of ASD. In this study, we sought to evaluate the effectiveness of behavioral interventions for this population by conducting a meta-analysis of published single-case research studies. A new metric for calculating effect size in single-case research, nonoverlap of all pairs, was utilized. In addition, the certainty of evidence, a system to evaluate research methodology, was applied to the reviewed articles. Forty-three articles were identified in the study. Results suggested that the behavioral interventions in the areas of academic skills, adaptive skills, problem behavior, phobic avoidance, social skills, and vocational skills have medium-to-strong effect sizes. Medium-to-high confidence in findings was noted for 81% of the studies in the meta-analysis; however, three-fourths of the reviewed studies did not include treatment integrity, which may affect the ability to draw conclusions about the effectiveness of the interventions. Overall, the evidence is promising for the use of behavioral interventions for this population; however, additional research and dissemination are needed to fill the gap between research and practice in order for practitioners to meet the increasing demand as individuals with ASD age.