Objective: Longitudinal studies have shown a strong association between ADHD and criminal behavior. The aims of this study were to examine the prevalence of adult ADHD in a high-security facility and to investigate the relationship between ADHD symptoms and psychopathic traits in criminal offenders. Material and Methods: Participants were recruited between October 2015 and January 2016 among the inmates from a high-security all-male correctional facility in Portugal. Self-report scales were used to measure ADHD symptoms, current psychopathology, and psychopathic traits. An interview was conducted to collect sociodemographic data, along with information about adverse childhood experiences, substance use, and criminal record. The statistical analysis was conducted using STATA 13 with the significance level set at .05. Results: From the final sample of 101 male prisoners, 17 participants (16.8%) screened positive for ADHD. Participants positive for ADHD had higher levels of general psychopathology and psychopathic traits. Psychopathic traits remained significantly higher in ADHD offenders after controlling for age, substance abuse, and early childhood adversities (OR = 1.07, p = .002) and this was explained by differences in the Meanness subscale (OR = 1.11, p = .013). Discussion/conclusions: The present study shows that ADHD symptoms are highly prevalent among offenders and might have a modulating effect on the course of delinquent behavior. The cross-sectional association between ADHD symptoms and psychopathic traits reinforces the previously documented correlation between these disorders and can lead to a better understanding of the prevalent criminal behavior in ADHD patients.