Objective: This study investigated whether the associations of own and partner intra- and extradyadic stress with relationship satisfaction vary based on both partners' Big Five personality traits among parents. Background: Previous research has shown that relationship satisfaction during parenthood often decreases, and increased stress is linked to lower relationship satisfaction; yet these associations differ widely between individuals. Personality traits may explain such individual differences. However, research is scarce on how they interact with intra- and extradyadic stress, especially during critical periods such as parenthood, and how these dynamics affect both parents' relationship satisfaction. Method: Using the Actor-Partner Interdependence Model, this study examined prospective associations between own and partner intra- and extradyadic stress and relationship satisfaction and whether the Big Five personality traits moderate these associations. Data were drawn from the German cohort study DREAM and included 1961 mothers and 1485 fathers, corresponding to 2021 couples in total, with data from both partners available for 1455 couples. Results: Higher levels of own and partner intradyadic stress were associated with lower own relationship satisfaction. The negative association between own intradyadic stress and own relationship satisfaction was stronger among more conscientious parents and weaker among parents with more open partners. Likewise, partner intradyadic stress was less strongly associated with lower own relationship satisfaction among more open parents. In contrast, partner extradyadic stress predicted higher own relationship satisfaction only among open parents. Conclusion: Our results highlight the need to consider both partners' personalities in parenting stress management interventions as well as the specific contextual factors of stress parents face.