Objective: This study aimed to examine the association between parental socioeconomic status (SES) and children's physical activity (PA) behaviour during after-school hours. Design: Cross-sectional study. Methods: Participants included 663 schoolchildren (aged between 10 and 13 years) and their parents from nine primary schools in Hong Kong. Parental SES status and the after-school PA levels of children were assessed using self-administered questionnaires. Analysis of variance (ANOVA) was conducted to examine the association between parents' education and income level and the type of activities in which children participated during after-school hours. Results: Results indicated that children with mother in the higher education group engaged in fewer time blocks of sports-type activity (high = 0.72 time blocks; medium = 1.04 time blocks; low = 1.17 time blocks). Compared with their counterparts, children with mothers in the higher income group were more engaged in sedentary activity (high = 5.33 time blocks; medium = 5.02 time blocks; low = 4.73 time blocks) within the eight 30-minute time blocks during after-school hours. Conclusion: The study describes how parental SES influences children's after-school PA participation and highlights the importance of including a family perspective when designing a PA promotion programme targeting after-school hours.