Given the persistent social communication challenges faced by children with autism spectrum disorder (ASD) and the need to integrate play into peer-mediated intervention (PMI) frameworks within general education settings, this systematic review synthesizes evidence on the effects of play-based peer-mediated interventions (PB-PMI) on social, play, behavioural, and mental health outcomes in children with ASD. Following PRISMA guidelines, a systematic search of Scopus, Web of Science, and ERIC databases (2016–2025) identified 19 studies. Findings revealed that 89.47% of studies reported improved social communication, 42.11% documented gains in play skills, and 31.58% reported benefits in behavioural regulation and mental health. Intervention effects were moderated by age, language ability, cognitive level, and co-occurring conditions. However, only 58% of studies reported intervention fidelity, 37% assessed social validity, and 63% evaluated generalisation or maintenance with limited depth.