There has been an abundance of research in recent years on mindfulness, including mindfulness within individuals seeking substance use treatment. However, to date, there has been no research on whether trait mindfulness is associated with increased aggression among individuals seeking substance use treatment. Past research has demonstrated that individuals in substance use treatment evidence higher levels of aggression than non-substance abusers, and preliminary research has shown that trait mindfulness is inversely associated with aggression in non-substance-use treatment-seeking populations. The current study examined whether trait mindfulness was associated with aggression among men seeking residential substance use treatment (N = 116). Results demonstrated that lower trait mindfulness was associated with increased aggression (physical, verbal, and aggressive attitude). Moreover, this relation held for both verbal aggression and aggressive attitude after controlling for alcohol use, drug use, and age, all known predictors of aggression. Findings provide the first evidence that mindfulness is negatively associated with aggression among men in substance use treatment, which could have important implications for intervention. That is, mindfulness-based interventions may prove helpful for the treatment of both substance use and aggression.