Objective Substance use disorders (SUDs) have risen substantially, especially in developing nations, and has become one of the world's most significant public health and socioeconomic challenge. This study aimed to determine the prevalence and patterns of substance use disorders among patients admitted at the main psychiatric hospital, Sbrana Psychiatric Hospital in Botswana. Method The study design was descriptive cross-sectional, involving patients admitted to a psychiatric hospital. SUDs were assessed using the Alcohol, Smoking, and Substance Involvement Screening Test (ASSIST) v13.0. Bivariate analysis was conducted to determine the association between socio-demographic characteristics and SUDs. Multivariate logistic regression was performed to rule out the effect of confounders. Results The final analysis involved one hundred and one (101) participants. Participants ages ranged from 18-60 years with over half (59.4%) being male. Majority of participants (85.1%) had ever used studied substances; with 63.4% found to have substance use disorders. The most frequently used substances were tobacco, cannabis, alcohol and cocaine at 58.4%, 42.6%, 34.7% and 12.8% respectively. SUDs were more common among participants who were male, younger age of 18–35 years, single, tertiary education and unemployed. Conclusions There is a high prevalence of substance use and SUDs among psychiatric patients in Botswana. Structured screening and interventions to manage substance use disorders should be part of routine care at the psychiatric health care facilities in Botswana. Consideration should be given to screening all patients admitted to psychiatric hospitals in Botswana for SUDs using the ASSIST.