The Samenta study was conducted in 2009 in the Greater Paris area to estimate the prevalence of psychiatric disorders in homeless people. A cross-sectional survey was performed with a three-stage random sample of homeless people (n = 859), including users of day services, emergency shelters, hot meal distribution, long-term rehabilitation centres, and social hotels. Information was collected by a lay interviewer, using the Mini International Neuropsychiatric Interview, and completed by a psychologist through an open clinical interview. In the end, a psychiatrist assessed the psychiatric diagnosis according to the International Statistical Classification of Diseases and Related Health Problems (ICD, 10th revision). One third of homeless people in the Paris area had at least one severe psychiatric disorder (SPD): psychotic disorders (13%), anxiety disorders (12%), or severe mood disorders (7%). One in five was alcohol-dependent and 18% were drug users. Homeless women had significantly higher prevalence of anxiety disorders and depression compared to men, who were more likely to suffer from psychotic disorders. Homeless people of French origin were at higher risk of SPD, as well as people who experienced various adverse life events before the age of 18 (running away, sexual violence, parental disputes, and/or addictions) and those who experienced homelessness for the first time before the age of 26. The prevalence rates of the main psychiatric disorders within the homeless population of our study are consistent with those reported in other Western cities. Our results advocate for an improvement in the detection, housing, and care of psychiatric homeless people.