Background There is paucity of research about contexts underlying vulnerability to risky sexual behavior and severe mental illness (SMI) among adults, irrespective of their HIV status in Uganda. Most studies done on vulnerability to risky sexual behavior (RSB) have often focused on young people and those living with HIV. Understanding factors that affect vulnerability to RSB in severe mental illness, taking into consideration the individual, carers and mental health specialists views is important in planning preventive interventions. We explored the socio-cultural factors and practices as well as social network structures that underlie SMI and vulnerability to RSB in an adult population.Methods We did a cross-sectional qualitative study at Butabika and Masaka hospitals. We collected data from a total of 32 participants; 12, 8 and 12, semi-structured interviews for persons living with SMI, carers and mental health specialists respectively. The interviews were tape-recorded and transcribed verbatim. We followed framework analysis principles, to manage, analyze and interpret the data. We used illuminating verbatim quotations to illustrate major findings.Results Risky sexual behavior was noted to be common among persons with severe mental illness; and a growing problem mostly understood in the context of HIV/AIDS and unwanted pregnancies, and sexual encounters with strangers. The problem was largely attributed to the compromised sense of judgement due to the underlying morbidity, abnormally high libido, poverty, desperation and inadequate social support associated with severe mental illness. Sexual starvation due to stigma/discrimination and long periods of hospitalization was also reported fuel irresponsible and risky sexual acts. Respondents also cited some practices and systemic weaknesses within the hospitals that inadvertently expose patients to RSB such as stripping patients going to the seclusion room naked, and inadequate monitoring of patients. The limited scope in understanding of the problem was thought to hamper efforts to address the problem. Unfortunately, there were no reports of health education initiatives specifically to address the problem.Conclusions The study revealed that risky sexual behavior is common among persons with severe mental illness, and identified several moderating factors. The study further highlighted the need to integrate sexual and reproductive health into mental health services.