The participation of patients in their treatment and care is perceived as desirable; however, patients with mental illnesses experience limited opportunities to participate in their own care. As nurses play a key role in taking care of patients with mental illnesses, this study aimed to investigate how nurses within psychiatric care settings experience patients’ participation and howthey act to increase it. Semi-structured interviews were conducted with eight registered nurses, four of whom worked in apsychiatric institutional care setting and four of whom worked with outpatients. Data were analysed using a qualitative content analysis approach. The analysis identified a theme: a caring relationship is a prerequisite for patient participation. This theme wa sfurther developed through five subthemes. The findings illustrate critical aspects of the caring relationship by which the nurse–patient relationship can either facilitate or impede patient participation. A caring relationship builds trust and increases the patient’s sense of responsibility for their own condition. Because patients with mental illness are cared for in many different contexts, the results of the present study have implications for a broad range of healthcare environments.