Objectives/Background: Insomnia is a serious condition that affects over 60% of the prison population and has been associated with aggression, anger, impulsivity, suicidality, and increased prison health care use. Nonpharmacological interventions for prison inmates are scarce despite the high prevalence and significant consequences of insomnia among those incarcerated. The aim of the present study was to examine the preliminary efficacy and effectiveness of a one-shot session of cognitive behavioral therapy for insomnia (CBT-I) for prison inmates with acute insomnia in an open trial. Method/Participants: The intervention consisted of one 60-70 min session of CBT-I and a self-management pamphlet. A consecutive series of 30 adult male offenders with acute insomnia from a UK prison completed measures of prospective sleep (daily sleep diary), insomnia symptoms severity (Insomnia Severity Index), and mood symptoms (Patient Health Questionnaire and General Anxiety Disorder) one week before and four weeks after receiving the intervention. Results: Pairwise t-tests revealed that a single shot of CBT-I was effective in reducing the severity of insomnia in adult male offenders (t = [29], 12.65, p < 0.001). Further, the results demonstrated moderate to large effect sizes for reductions in depressive (d RM = 0.77) and anxious (d RM = 0.83) symptoms, as well as insomnia severity (d RM = 2.35). Conclusions: A single-shot session of CBT-I is effective in managing acute insomnia and mood (depression, anxiety) symptoms in adult male prison inmates. Future research should focus on testing if the single-shot CBT-I intervention can be implemented and disseminated in other settings and populations (e.g., female and juvenile or youth offenders). An estimated 11-81% of prison inmates report insomnia symptoms (Dewa, Kyle, Hassan, Shaw, & Senior, 2015). The wide variation among studies may be due to different criteria used to define insomnia as well as different measures used to assess insomnia symptoms. For example, requests for hypnotics were used as an indicator of insomnia in one study (10.9%; Kjelsberg & Hartvig, 2005), whereas other studies, with much higher prevalence rates, used clinical screening instruments or general health questionnaires with one or two sleep-related items embedded within (e.g., 74%, Singleton, Meltzer, & Gatward, 1998; 71. 6%, O'Brien, 2001). The only study to date that has used an insomnia-specific measure, the Sleep Condition Indicator (Espie et al., 2014), suggests an overall prevalence rate for Insomnia Disorder of 61.6%, with women significantly more likely to report insomnia, in prison, compared to men (Dewa, Hassan, Shaw, & Senior, 2017). The consequences of insomnia in prison inmates include aggression, anger, impulsivity, and increased prison health care use (