Existing interventions to reduce self-harm in adolescents admitted to psychiatric wards are usually focused on individual psychological treatments. However, the immediate ward environment in which treatment takes place is an important factor in the success of the treatment and can also influence the likelihood of self-harming behaviours. The aim of the current study was to evaluate changes made to a psychiatric ward environment on incidence of self-harm in adolescents. A quasi-experimental interrupted time series study was conducted on one child and adolescent psychiatric ward. An intervention was developed alongside staff and patients to address the high incidence of self-harm on weekday evenings on the ward. The intervention components involved adding a regular twilight shift (3-11 pm) for nursing staff and introducing a structured evening activity programme on the ward. A segmented regression analysis of an interrupted time series found that the rate of self-harm per 100 bed days was already declining at baseline and continued to decline post-intervention, but the rate of decline was not significant (p = 0.415). However, the proportion of patients self-harming was increasing at baseline and significantly reduced post-intervention (p = 0.001), and this reduction was significantly larger in the evenings (p = 0.004) compared to other times of day (p = 0.09). A tailored intervention targeting the psychiatric ward environment helped to reduce the proportion of adolescents self-harming on the ward. An interrupted time series analysis should be considered for future interventions making changes to health systems over time. © 2020. The Author(s).