The current study is a randomized clinical trial evaluating the efficacy of Focused Playtime Intervention (FPI) in a sample of 70 children with Autism Spectrum Disorder. This parent-mediated intervention has previously been shown to significantly increase responsive parental communication (Siller et al. in J Autism Dev Disord 43:540-555, 2013a). The current analyses focus on children's attachment related outcomes. Results revealed that children who were randomly assigned to FPI showed bigger increases in attachment-related behaviors, compared to children assigned to the control condition. Significant treatment effects of FPI were found for both an observational measure of attachment-related behaviors elicited during a brief separation-reunion episode and a questionnaire measure evaluating parental perceptions of child attachment. The theoretical and clinical implications of these findings are discussed.