Substance use disorders (SUD) are common among people with psychotic disorders and are associated with many negative consequences. Understanding the reasons for substance use in this population may allow for the development of more effective prevention and intervention strategies. We examined reasons for tobacco, alcohol, or cannabis use among people with psychotic disorders. Sixty-four participants with a diagnosed psychotic disorder completed a self-report reasons for use questionnaire. A subset of eight participants completed semi-structured qualitative interviews. Both the qualitative and quantitative data indicated that reasons for use of tobacco, alcohol, and cannabis differed considerably. Tobacco was primarily used for coping motives, alcohol for social motives, and cannabis for pleasure enhancement motives. Prevention and intervention strategies targeting coexisting psychotic disorders and SUD may improve in effectiveness if they address the perceived beneficial effects of tobacco use, the strong social pressures influencing alcohol use and if they encourage cannabis users to seek alternative pleasurable activities.