Decision-making capacity (DMC) among psychiatric inpatients is a pivotal clinical concern. A review by Okai et al. (2007) suggested that most psychiatric inpatients have DMC for treatment, and its assessment is reliable. Nevertheless, the high heterogeneity and mixed results from other studies mean there is considerable uncertainty around this topic. This study aimed to update Okai's research by conducting a systematic review with meta-analysis to address heterogeneity. We performed a systematic search across four databases, yielding 5351 results. We extracted data from 20 eligible studies on adult psychiatric inpatients, covering DMC assessments from 2006 to May 2022. A meta-analysis was conducted on 11 papers, and a quality assessment was performed. The study protocol was registered on PROSPERO (ID: CRD42022330074). The proportion of patients with DMC for treatment varied widely based on treatment setting, the specific decision and assessment methods. Reliable capacity assessment was feasible. The Mini-Mental State Examination (MMSE), Global Assessment of Function (GAF), and Brief Psychiatric Rating Scale (BPRS) predicted clinical judgments of capacity. Schizophrenia and bipolar mania were linked to the highest incapacity rates, while depression and anxiety symptoms were associated with better capacity and insight. Unemployment was the only sociodemographic factor correlated with incapacity. Assessing mental capacity is replicable, with most psychiatric inpatients able to make treatment decisions. However, this capacity varies with admission stage, formal status (involuntary or voluntary), and information provided. The severity of psychopathology is linked to mental capacity, though detailed psychopathological data are limited.