In order to address gaps in gratitude intervention research, a 3-armed trial was conducted. The aim of the study was to evaluate the effects of a 6-week gratitude intervention in people with suboptimal levels of well-being on gratitude, mental health and positive emotions. Participants were recruited in the Dutch population and randomly assigned to the 6-week gratitude intervention (n = 73), an active control condition (kind acts for yourself) (n = 73) or a wait-list control condition (n = 71). Participants completed assessments at baseline, 2 and 4 weeks during the intervention, post-test, 6-week and 6-months follow-up. Changes in outcomes were analyzed using multilevel growth curve modeling. Significant linear and quadratic increases in mental well-being, gratitude and positive emotions were found and a linear increase in positive relations relative to wait-list control. Compared to the active control condition, quadratic increases in mental well-being and dispositional gratitude, and linear increases in positive emotions and positive relations were found. The current findings suggest that a 6-week gratitude intervention may be a promising approach to improve mental health. To explore the role of positive emotions, analyses about the efficacy of the intervention on positive emotions and their mediating role will also be presented.