Sexual and relationship well-being are strongly intertwined in couple relationships. However, relationship quality is generally examined through relationship satisfaction only, neglecting the importance of sexual satisfaction as an aspect of relationship quality for many couples. Moreover, considering the role of interpersonal processes, such as intimacy, in relationship quality seems essential to better understand couples' optimal functioning. Although the Vulnerability-Stress-Adaptation Model posits bidirectional associations between interpersonal processes and relationship quality, few studies have tested such models. This dyadic study examined the longitudinal associations between the process of intimacy and sexual and relationship satisfaction in 145 long-term mixed-gender/sex couples. Participants completed online questionnaires three times over 13 months. A cross-lagged panel model revealed bidirectional associations between intimacy behaviors and both sexual and relationship satisfaction. Findings suggest that intimacy behaviors could foster both sexual and relationship satisfaction in couples, and that higher sexual and relationship satisfaction could also promote intimacy behaviors over time. These results highlight that satisfaction may not only be an outcome of positive interpersonal processes, but also a facilitator of these interpersonal processes over time. This study underscores the need to consider the complex interplay of interpersonal processes and satisfaction to foster relationship quality in long-term couples over time.