Introduction <p>This 1-year follow-up study investigated the cross-sectional and prospective associations of (1) personal and environmental factors (demographic characteristics, attention-deficit/hyperactivity [ADHD] diagnosis, internalizing and externalizing problems, hostility, impulsivity, and the quality of relationships with friends and family) with Internet addiction and of (2) Internet addiction with various mental health problems (depression, suicidality, and nonsuicidal self-harm) among adolescents. In addition, the moderating effects of ADHD on these associations were analyzed.</p> Methods <p>Data were collected from 349 adolescents (176 with ADHD and 173 without ADHD at baseline). In total, 249 participants (71.3%; 122 with ADHD and 127 without ADHD) completed the 1-year follow-up assessment.</p> Results <p>At baseline, impulsivity (e.g., lack of perseverance and self-control and inability to plan), hostility (expressive hostility behavior), age, and low satisfaction with relationships with friends were significantly associated with Internet addiction severity. At follow-up, impulsivity (e.g., lack of perseverance and self-control), hostility (expressive hostility behavior), and Internet addiction severity at baseline were significantly associated with Internet addiction severity. In addition, Internet addiction severity at baseline was cross-sectionally associated with depression. Notably, ADHD moderated the cross-sectional association between Internet addiction severity and nonsuicidal self-harm at baseline. Furthermore, Internet addiction severity at baseline prospectively predicted suicidality at follow-up.</p> Conclusion <p>Internet addiction can predict suicidality among adolescents. Personal and environmental factors should be integrated into the assessment of Internet addiction in adolescents.</p>