In adult couples, stress emerging outside the relationship (extra-dyadic stress) spills over into the relationship evoking conflict between the partners (intra-dyadic stress), which itself has a negative effect on relationship satisfaction over time. This detrimental effect of stress spillover can be buffered by adequate dyadic coping (DC) skills of both partners. The current study aimed at replicating these effects in a sample of 96 Swiss adult couples and 124 Swiss late adolescent (non-cohabitating) couples, who were in a serious relationship for at least 1 year. We examined (a) whether extra-dyadic stress is associated with intra-dyadic stress and relationship satisfaction and (b) whether the potential buffering effect of DC on these associations already exists in this important developmental phase. Contrary to the adult sample, for which indirect actor and partner effects were significant, we found a marginally significantly negative indirect actor effect of extra-dyadic stress on relationship satisfaction via intra-dyadic stress in late adolescents only for participants experiencing high extra-dyadic stress and low support from the partner. DC moderated the spillover between extra-dyadic and intra-dyadic stress for late adolescents. This study highlights the importance of investigating dyadic processes in romantic relationships in late adolescence to gain an insight into these relationships and the associated developmental processes.