Relationships with parents and teachers are crucial to a child's socialization. However, little is known about the transactional processes between parent-child and teachers-student relationships and their mediating mechanisms. This short-term longitudinal study examined bidirectional relations between positive parent-child and teachers-student relationships, and the potential mediating role of positivity within these relations. There were 3,450 Chinese children (44.8% girls; M[subscript age] = 10.93 years) who participated in a four-wave longitudinal study, spaced 6 months apart. The random intercept cross-lagged panel modeling found: (a) both father-child and mother-child relationships directly predicted teachers-student relationships, and vice versa; (b) positivity functioned as a mediator in bidirectional relations between parent-child and teachers-student relationships. These results support a transactional spillover effect between parent-child and teachers-student relationships, suggesting Chinese children may become caught in a virtuous cycle either directly or indirectly via their positivity.