No evidence exists in the literature concerning the prevalence of insomnia and its associated risk factors among prison inmates in Taiwan. The aim of the present study was to determine the prevalence and factors associated with insomnia among inmates in a large prison in Taiwan. A cross‐sectional anonymous questionnaire survey was conducted at a prison. The participants were 1490 male inmates. Participants completed the self‐reported Insomnia Severity Index–Chinese version questionnaire, and the sociodemographic and psychological distress questionnaires for the study. Multivariate logistic regression was used to analyze and compare the differences between inmates with and without insomnia. The prevalence of insomnia was 26.9%. The study determined that anxiety, self‐rated health status, and religious beliefs were independent predictors of insomnia in male inmates. Given the adverse effect of insomnia and its social consequences, it is crucial to develop prevention programs to mitigate insomnia in inmates.