Introduction: Internet addiction is becoming a global problem and has been associated with mental health issues such as depression and anxiety. There is a strong need for local research, given the recent rise of mental health issues amongst children and adolescents in Malaysia, as well as lack of research in this field locally. Methods: This study assessed the prevalence of internet addiction, and its association with internetrelated factors, depressive symptoms, anxiety symptoms, and self-esteem, amongst selected secondary school students in the Klang Valley, Malaysia. In a cross-sectional study, 199 adolescents were conveniently chosen from six randomly selected secondary schools within Klang Valley. Subjects completed a demographic questionnaire, as well as Malay versions of the Internet Addiction Test (MVIAT), Children’s Depression Inventory (CDI), Depression Anxiety Stress Scales-21 anxiety subscale (DASS-21) and Rosenberg Self-Esteem Scale (RSES). Multiple logistic regression analysis was carried out. Results: This study found that 49.2% of students were addicted to the internet. Significant factors associated with internet addiction were spending longer hours online during the holidays (OR 1.20, CI 1.01-1.35), easy access to a computer (OR 4.01, CI 1.50 - 10.76), confiding on the internet (OR 4.07, CI 1.49-11.13), severe level of anxiety (OR 3.63, CI 1.04-12.73) and high depression scores (OR 1.10, CI 1.01-1.20). Contrary to previous studies, self-esteem was not found to be significantly associated with internet addiction. Conclusion: This study has important implications in relation to clinical practice, research, and policy-making in Malaysia.