Objective The present study investigated the beneficial effects of the resource‐oriented positive writing intervention resource diary (RD) on mental health variables among patients recently discharged from psychiatric inpatient treatment. Method Eighty‐nine patients were randomly assigned to either an intervention group completing RD over the course of 4 weeks (n = 45) or a control group receiving no intervention (n = 44). To measure changes in mental health, patients filled out a number of self‐report questionnaires on depression, emotion regulation, and resource activation before and after the intervention.  Results Participants completing RD had significantly lower depression scores than controls and reported an increased use of the functional emotion regulation strategy “reappraisal” 5 weeks after discharge. A decreased use of the dysfunctional strategy “expressive suppression” was found in the female subsample. No differences were found for resource activation. Conclusion These findings suggest that a resource‐oriented positive writing intervention has potential for stabilizing mental health after psychiatric discharge and could therefore present an economical alternative or addition to established aftercare programs.