ABSTRACT Group care workers of residential youth care settings face the challenge of creating a warm and involved treatment climate against the demands and restrictions of the treatment setting. We tested the effects of SafePath, a milieu-based intervention based on Schema Therapy principles, during the first year of implementation on two secure residential treatment units compared to two control units. Staff’s daily reports on 139 individual patients were coded on use of schema mode language (implementation check), occurrences of aggression (primary outcome) and repressive staff interventions. In addition, repeated questionnaires were filled out by patients (n = 87) on group climate and by staff (n = 50) on team functioning. Compared to the control units, SafePath units showed higher improvements in group climate and repressive interventions. Both SafePath and control units showed decreased aggression over time. Team functioning was consistently better in the SafePath units compared to the care-as-usual units from baseline through 12 months. In conclusion, a Schema Therapy based milieu as implemented with SafePath may contribute to a warm and supportive group climate with less repressive interventions in secure residential youth care.