Objective: To investigate the effectiveness of a guided, internet- and smartphone-based gratitude intervention on the transdiagnostic risk-factor ‘repetitive negative thinking’. The multicomponent intervention integrates a variety of gratitude exercises, targeting the cognitive, emotional and behavioural facets of gratitude. Method: Two hundred adults with pronounced repetitive negative thinking were recruited from the general population. Participants were randomly assigned to either a four-session guided gratitude intervention (n = 100) or waiting list (n = 100). The primary outcome was repetitive negative thinking three months after randomization, with exploratory assessments at six weeks and six months, the latter just for participants in the intervention group. Results: Following the intention-to-treat principle, by analyses of covariance (ANCOVA), the gratitude intervention group exhibited significantly lower levels of repetitive negative thinking than controls at three months, with d = 0.66, 95 % CI [0.37, 0.94] maintained at six-month follow-up. Significant and meaningful beneficial effects were observed in symptoms of depression (d = 0.42) and generalized anxiety (d = 0.38). These effects were notably stronger in intervention completers who finished at least three sessions. Conclusions: Results suggest that a multicomponent gratitude intervention is effective at reducing repetitive negative thinking. Multicomponent interventions may be a next step needed to fully realize the potential of gratitude interventions. Such interventions could expand the repertoire of transdiagnostic interventions, especially for repetitive negative thinking. Furthermore, due to its positive connotations, gratitude is a candidate for an indirect intervention aimed at reducing the burden of depression in the general population. Trial registration: The study is registered in the German Clinical Trial Register (approved primary register of the WHO) as DRKS00006825. The trial protocol can be assessed at: https://www.drks.de/