Patient participation is central in modern health care. However, it is a complex phenomenon that lacks a clear definition, and what constitutes participation varies depending on the context and theoretical perspective. It is known that patient participation in forensic psychiatric care is often rated as low by both patients and professionals, and it can be assumed that interventions to increase it are beneficial. In this process, management and staff could benefit from assessing perceived patient participation, and reliable and valid measurement instruments are essential. The aim of this study was to develop an instrument that could be used to measure experiences of participation in forensic psychiatric care from a patient perspective and test it for content validity. A definition of patient participation in forensic psychiatric care was formulated and operationalized in an instrument that an expert group, consisting of patients with ongoing care, evaluated for content validity. In total, 50 items were sorted into five different dimensions: to have good communication, to be involved, to have mutual trust, to trust the care, and to take responsibility. After psychometric testing, the instrument has the potential to become a tool to use in research, clinical work, and development work in the field of forensic psychiatric care. In addition to being used as a measure, the Patient Participation in Forensic Psychiatric Care can also be used to encourage a dialogue about their care and to make both patients and staff more aware of patient participation.