Welcoming a new child is a life-changing event that may deeply impact the mental health of parents, with parenting stress remaining one of the most commonly reported difficulties of the postpartum period. Parenting alliance, a specific component of the couple's relationship focused on parenting, has emerged as a key asset to deal with the stressful demands of early parenting. Although studies have confirmed that a sound parenting alliance is related to lower parenting stress, fathers are often understudied, and the reciprocal influences between coparents are still misunderstood. In addition, the potential influence of parents' history of childhood interpersonal trauma is still unclear. Since parents exposed to cumulative childhood interpersonal trauma (CCIT) are more likely to report early parenting difficulties and later adverse family outcomes, further investigation is needed into how CCIT may disrupt the relationship between parenting alliance and parenting stress. Using a dyadic longitudinal approach, this study examined the moderating role of CCIT in the association between parenting alliance and parenting stress. A randomly selected community sample of 958 parental couples with an infant completed online questionnaires across the first year postpartum. Results showed that higher parenting alliance predicted lower parenting stress 10 months later, with significant interdependence between coparents' functioning and dyadic associations. The strength of the associations varied based on parents' CCIT levels, with higher exposure weakening the positive influence of parenting alliance on parenting stress for mothers and fathers (R2 = 20.8%; 14.8%). These findings reveal the role of CCIT in shaping couple dynamics during postpartum and highlight the need for trauma-informed approaches in prevention and intervention efforts targeting the mental health of parents.