Background and aim: Having decision making capacity is central to the exercise of autonomy in mental health care. The objective of this scoping review is to summarize the evidence on the capacity of people with schizophrenia or bipolar disorder to make decisions about their treatment in real life to support medical practice. Methods: Systematic search of observational studies on the assessment of capacity of patients with schizophrenia, psychosis, or bipolar disorder to make healthcare and treatment-related decisions, conducted in any clinical setting published up to January 31, 2020 was performed. Free text searches and medical subject headings in English were combined in PubMed, Scopus, CINAHL, and PsycInfo. Publications were selected as per inclusion and exclusion criteria. The Newcastle-Ottawa Scale for observational studies was used to assess the quality of publications. Results: Thirty publications were reviewed. According to the Newcastle-Ottawa Scale criteria, the publications reviewed were good quality. Findings showed that more than 70% of schizophrenia and schizoaffective disorder outpatients understood treatment options at the point of making decisions about their illness and healthcare. Patients treated voluntarily had considerably better scores for decisional capacity than those treated involuntarily. The burden of psychiatric symptoms could compromise decisional capacity temporarily. Decision-making capacity improved over time from admission to discharge from hospital, and with treatment among psychiatry inpatients. Schizophrenia and bipolar disorder patients could be as competent as nonpsychiatric individuals in making decisions about their treatments in everyday life. Conclusions: This scoping review provides a body of evidence for healthcare professionals in need of assessing the capacity of schizophrenia and bipolar disorder patients for autonomously decide about their treatments. Decisional capacity judgements should consider variations in capacity over time and be based on the type of decision to be made, the severity of symptoms, and the specific phase of the mental disorder.