ABSTRACTParent–grandparent coparenting is increasingly prevalent worldwide, but whether and how parenting behaviours differ across generations and the influence of such potential discrepancies on child behaviour problems are still unclear. This study compares differences in parenting practices between parents and grandparents—specifically, harsh versus supportive parenting—and examines both the associations of these practices with child behaviour problems and the role of multigenerational coresidence in these associations. We recruited 404 parent–grandparent coparenting dyads from two highly industrialized urban cities in China. The results showed that coresiding grandparents tended to display less supportive parenting behaviours than parents. Parental harsh discipline and grandparental harsh discipline were both positively associated with child externalizing behaviour problems. Larger directional differences in harsh discipline between parents and grandparents were associated with more internalizing behaviour problems in their children. A larger absolute difference in self‐perceived supportiveness between parents and grandparents was negatively associated with children's internalizing behaviour problems only when parents and grandparents lived separately. This study suggests (grand)parenting style matters to children's externalizing behaviour problems, while the discrepancies between parenting and grandparenting practices have stronger influences on children's internalizing behaviour problems. These findings underscore the significance of promoting effective collaborative parenting in intergenerational families.