Insomnia has a negative impact on mental health and is a potential risk factor for impulsive and problematic behavior. This multicenter, cross-sectional study investigated the prevalence of insomnia and underlying and maintaining factors in a group of forensic psychiatric inpatients (N = 281). Insomnia severity, subjective sleep quality, sleep hygiene and erroneous cognitions about sleep were measured with the Insomnia Severity Index, Pittsburgh Sleep Quality Index, Sleep Hygiene questionnaire and Dysfunctional Beliefs and Attitudes about Sleep, respectively. Impulsivity was derived from risk assessment instruments routinely completed by trained professionals. Almost half of the patients (48.7%) appeared to suffer from insomnia. Particularly moderate-severe insomnia (26.7%) was significantly associated with worse sleep quality, poorer sleep hygiene, stronger endorsement of dysfunctional sleep cognitions and higher impulsivity scores. It can be concluded that insomnia is rather common in forensic psychiatric patients. Insomnia appears related to various sleep hygiene behaviors and sleep-related cognitions, and probably also to diverse situational and environmental factors as well as a lack of autonomy. Cognitive behavioral therapy for insomnia, with some adjustments specific for this population, may be an effective intervention, ameliorating sleep problems and possibly also emotional and behavioral dysregulation.