Background: Mental well-being could be promoted and protected by positive psychology (PP) based interventions. Such interventions may be appealing for people at risk of anxiety and depressive disorders, but health-economic evaluations are scarce. The aim was to examine the cost-effectiveness of a PP intervention. Methods: Participants with suboptimal levels of mental well-being were randomly assigned to an email guided PP-intervention (n = 137) or a wait-list control group (n = 138) with access to usual care (UC). At baseline and 6 months follow-up, data were collected on health care costs. Outcomes of interest were flourishing mental health and treatment response on anxiety and depressive symptoms. Results: Bootstrapped mean incremental cost-effectiveness ratios were €2359 ($2899) for flourishing, €2959 ($3637) for anxiety and €2578 ($3168) for depression, suggesting appreciable health gains for low additional costs. At a willingness to pay ceiling of €10,000 ($12,290) for a treatment response, the probability that the intervention is deemed cost-effective ranged between 90 and 93%. Conclusions: The guided PP intervention appears to be a promising strategy as seen from both a public health and a health-economic perspective, especially when there is some willingness to pay. When the PP-intervention is scaled up, then outcome monitoring is recommended to better guarantee the longer term cost-effectiveness of the intervention.