Risk assessment instruments are widely used to predict risk of adverse outcomes, such as violence or victimization, and to allocate resources for managing these risks among individuals involved in criminal justice and forensic mental health services. For risk assessment instruments to reach their full potential, they must be implemented with fidelity. A lack of information on administration fidelity hinders transparency about the implementation quality, as well as the interpretation of negative or inconclusive findings from predictive validity studies. The present study focuses on adherence, a dimension of fidelity. Adherence denotes the extent to which the risk assessment is completed according to the instrument’s guidelines. We developed an adherence measure, tailored to the ShortTerm Assessment of Risk and Treatability: Adolescent Version (START:AV), an evidence-based risk assessment instrument for adolescents. With the START:AV Adherence Rating Scale, we explored the degree to which 11 key features of the instrument were adhered to in 306 START:AVs forms, completed by 17 different evaluators in a Dutch residential youth care facility over a two-year period. Good to excellent interrater reliability was found for all adherence items. We identified differences in adherence scores on the various START:AV features, as well as significant improvement in adherence for those who attended a START:AV refresher workshop. Outcomes of risk assessment instruments potentially impact decision-making, for example, whether a youth’s secure placement should be extended. Therefore, we recommend fidelity monitoring to ensure the risk assessment practice was delivered as intended.