This study evaluated (a) the association between Internet addiction and adult ADHD; (b) which one of inattention, hyperactivity, or impulsivity was most associated with Internet addiction; and (c) whether gender modulates the association between Internet addiction and inattention, hyperactivity, or impulsivity among college students. A total of 2,793 students (937 male and 1,843 female) were recruited from eight colleges in Taiwan, and they all completed the Chen Internet Addiction Scale, Adult ADHD Self-Report Scale, and demographic data. The results demonstrated that adult ADHD was associated with Internet addiction. Attention deficit was the most associated symptom of Internet addiction, followed by impulsivity. Furthermore, the association between attention deficit and Internet addiction was more significant among female college students. Adult ADHD should be surveyed and treated among college students to decrease the vulnerability to Internet addiction, and strategies to prevent Internet addiction should be provided for college students with ADHD, especially for females.