Based on the fact that most research drawn from the minority stress theory on the association between minority stress and relationship satisfaction has focused on an individual perspective rather than a dyadic perspective, and the limited evidence of the systemic transactional model (STM) explore the topics of same-sex couples. This study aims to combine both theories to examine the association between perceived discrimination and relationship satisfaction among lesbian, gay, and bisexual (LGB) people on both partners and test the potential mediating role of dyadic stress and sex difference in this association. Using an actor-partner interdependence mediation model (APIMeM), we analyzed data including a sample of 241 LGB couples (133 female and 108 male same-sex dyads). Results showed that perceived discrimination had no direct actor-partner effects on relationship satisfaction. APIMeM revealed significant indirect partner effects from perceived discrimination on both individuals’ and their partners’ relationship satisfaction through the partner’s dyadic stress. Additionally, the effect of personal dyadic stress on a partner’s relationship satisfaction was stronger for women compared to men. These findings demonstrated the utility of the minority stress theory and STM framework for understanding the risk of stressors in damaging LGBs’ romantic relationships. Couple interventions should integrate knowledge from a dyadic perspective with attention to sex differences.