ObjectiveThe study tested the grandmother role difference hypothesis (GRDH) in Chinese three‐generation families by investigating the effects of mothers' and grandmothers' perceptions of the coparenting relationship on child adjustment and the potential mediating effect of parenting self‐efficacy.BackgroundMother–grandmother coparenting has become the prevalent parenting mode in urban China, yet previous studies have failed to recognize the differences between the grandmothers by lineage. By reviewing biological theories and sociological contexts, this study proposed the GRDH. The hypothesis posited that in addition to maternal grandmother's benefits on grandchildren over paternal grandmother due to biological drives, maternal and paternal grandmothers exert their effect on child outcomes disparately by developing different relationships with the mother in coparenting.MethodIn total, 409 mother–grandmother coparenting families were recruited. Two identical structural equation models were examined separately for maternal and paternal grandmother groups.ResultsThe results of multigroup analysis showed a stronger direct effect by maternal grandmother's perception of coparenting relationship than paternal grandmother on child social competence. The relation between maternal parenting self‐efficacy and child social competence was stronger in households with paternal grandmothers. Furthermore, the mothers' evaluation of the coparenting relationship had a significant spillover effect on the paternal grandmother's parenting self‐efficacy only. The findings also showed the exclusive mediating effect of maternal parenting self‐efficacy linking the mother's perception of the coparenting relationship to child problem behavior in the mother–paternal grandmother group only.Conclusion and implicationsThe results overall validated the GRDH while providing further contextual information on the roles of mothers and grandmothers in different types of coparenting families.