We evaluated the utility of a brief, seven-item, teacher-rated Peer Social Maturity Scale (PSMAT). In Study 1, teachers of 138 Australian children (ranging from 5 to 8 years and 5 months old) in kindergarten and Grades 1 and 2 rated their pupils' social maturity using the PSMAT and their classroom social skills via the Social Skills Rating System (SSRS). The PSMAT showed excellent internal consistency and a significant overall correlation with the SSRS social skills scale. Study 2 involved a new longitudinal sample of children who were rated by different classroom teachers in kindergarten and Grades 1 and 2 of full-time primary school. Consistent with Study 1, at all three time points, PSMAT scores were highly correlated with SSRS social skills scores. PSMAT scores also correlated significantly with peer-rated social preference. These studies confirm that the PSMAT is a reliable and valid assessment of children's social maturity within their classroom peer groups.