Illegal behaviors have been reported in gambling disorder (GD) as well as in individuals with attention deficit hyperactivity disorder (ADHD). Likewise, ADHD symptomatology has been observed in patients with GD. We aimed to examine the association between ADHD symptomatology, personality traits and impulsivity in a sample of treatment-seeking patients with GD (n = 204) with and without a criminal report. The subjects were evaluated before starting treatment using different self-administered instruments. Among those who had committed an illegal act, two groups were made depending on whether or not the criminal conduct had legal repercussions (n = 64 without legal consequences; n = 32 with legal consequences). Structural equation modeling (SEM) was used to explore the interrelationships between personality traits, ADHD, impulsivity levels, gambling-related criminal behavior and other gambling-related factors. Greater ADHD symptomatology was found in patients with reported illegal behaviors, as well as higher impulsivity levels. Those individuals who presented legal consequences due to the criminal behavior showed higher impulsivity levels and harm avoidance and lower persistence and cooperativeness. Our findings uphold that patients with GD and ADHD symptomatology have greater impulsivity and are more vulnerable to committing an illegal act. Therefore, specific harm-prevention interventions and treatment approaches are needed for this population.