Background: Temperamental effortful control involves the voluntary control of attention and behavior. Deficits in effortful control put children at risk for developing externalizing behavior problems. Coparenting behavior, or the extent to which parents support or undermine each other's parenting efforts, has also been identified as an important correlate of children's socioemotional adjustment. The present study tested whether coparenting behavior moderated longitudinal relations between preschool children's effortful control and their externalizing behavior. Methods: Ninety-two families (mother, father, 4-year-old child) participated. Parents' coparenting behavior was observed during family interaction, and children's effortful control was rated by parents. At that time and one year later, mothers and teachers reported on children's externalizing behavior. Results: Supportive coparenting behavior moderated longitudinal relations between children's effortful control and mothers' and teachers' reports of their externalizing behavior, even when taking into account initial levels of externalizing behavior. Conclusions: Effective coparenting served as a buffer for children, such that when parents displayed high levels of supportive coparenting behavior, the link between low effortful control and increases in externalizing behavior was not observed.