This study aimed to examine the effect of attachment security in 17-month-olds on their peer play interactions and behavioural problems at ages 4 and 6 years, respectively, in Korea. By employing structural equation modelling, we analyzed the data of 183 children and their mothers, which were extracted from the Panel Study on Korean Children (2009-2013). The results indicated that higher attachment security at 17 months was associated with positive play interactions with peers at age 4, which in turn led to fewer internalizing and externalizing problems at age 6. Conversely, lower attachment security at 17 months was related to negative play interactions with peers at age 4, which in turn led to more externalizing problems at age 6. These findings highlight the importance of early infant-mother attachment security to enhance peer competence and protect against behavioural problems in early childhood.