Patients using mental health services are vulnerable to abuse and exploitation, yet there is no consensus on assessment for values-based recruitment (VBR) in this setting. VBR requires evaluating attitudes, knowledge and traits relevant to the delivery of professional and effective care. Structured interviews are one approach to VBR but are resource intensive to deploy. In contrast, situational judgement tests (SJT) are generally valid predictors of job performance and can be delivered more cost-effectively, and at scale. This project developed and validated a SJT that aimed to assess one’s knowledge of professionalism in a mental health services context. A mixed methods programme of work was conducted, incorporating a rapid systematic review to define professionalism in mental health services, a qualitative study exploring stakeholder perspectives, and a quantitative study that assessed the criterion-related validity of the resulting SJT. The literature review included 70 articles and resulted in two operational definitions of professionalism. The subsequent qualitative study incorporated interviews and focus groups with 56 patients, carers and staff members from a range of professions, who facilitated the development of the SJT. Finally, a pilot study, which involved 170 mental health professionals, was conducted to evaluate the validity of the SJT. The SJT scores validly predicted workplace supervisor ratings of professionalism and effectiveness for nurses and allied health professionals. Its predictive ability was comparable to that previously reported for face-to-face interviews. In conclusion, the SJT can be reliably deployed in mental health services to assist values-based recruitment by identifying candidates who are unable to demonstrate reasonable levels of knowledge related to professional behaviours. This novel tool can support our efforts to reduce the risk of patient abuse and reported instances of malpractice in mental health services.