A growing body of research has documented positive outcomes of gratitude in personal and interpersonal domains. To uncover the dynamic process of gratitude and relational well-being, we examined the interplay of grateful disposition, grateful mood, and grateful expression in ongoing close relationships. Hong Kong Chinese couples (n = 100) participated in a three-wave study across three consecutive weeks. Adopting the Actor-Partner Interdependence Model, we found that at Time 1, grateful disposition not only predicted one's own grateful mood but also the perceived grateful mood of one's spouse, both of which predicted marital satisfaction. At Time 2, the couples were randomly assigned to two conditions over 2 weeks: having one spouse keeping a private gratitude journal or overtly expressing gratitude to the other. Couples' grateful mood increased at Time 3, indicating the effectiveness of both interventions. However, the resulting changes in marital satisfaction differed for the beneficiaries (enactors) and benefactors (targets), such that husbands who perceived their wife's expressed gratitude as less sincere declined in their marital satisfaction. The results reveal the boundary conditions in evaluating expressions of gratitude and improvement of relationships and provide implications for social exchange and couple therapy.