Purpose: Decisional conflict refers to the degree to which patients are engaged in and feel comfortable about important clinical decisions. Until now, the concept has received little attention in mental health care. We investigate the level of decisional conflict in mental health care and whether this is influenced by socio-demographics, treatment setting, diagnoses, and locus of control. Methods: Cross-sectional study among 186 patients in Dutch specialist mental health care using the Decisional Conflict Scale, which measures five dimensions of decisional conflict: information, support, clarification of values, certainty, and decisional quality. Descriptive statistics and forward stepwise linear regression analyses were used. Results: Patients report relatively high levels of decisional conflict, especially those with more external locus of control. Having a personality disorder and higher education also increases decisional conflict on the dimensions support and clarification of values, respectively. Less decisional conflict was experienced by patients with psychotic disorders on the dimension certainty and by women on the information domain. Conclusions: Decisional conflict is common among patients in specialist mental health care and is very useful for assessing the quality of clinical decision making. Measuring decisional conflict and knowledge about influencing factors can be used to improve patients’ participation in clinical decision making, adherence to treatment and clinical outcomes.