Background: Digital addiction is an emerging public health concern among adolescents, and India, with the world’s largest adolescent population, is particularly affected. This study estimated the prevalence and pattern of digital addiction among school-going adolescents and identified its socio-demographic and behavioural correlates. Methods: This cross-sectional analysis used baseline data from a cluster randomized controlled trial conducted in eight secondary schools (four government, four private) in India. A total of 400 adolescents aged 13–16 years (grades 8–10) were assessed using Young’s Internet Addiction Test (IAT-20), the Smartphone Addiction Scale–Short Version (SAS-SV), a socio-demographic and digital-use questionnaire, and the Depression Anxiety Stress Scales-21 (DASS-21). Factors associated with internet addiction were examined using multivariate logistic regression. Results: The mean age of participants was 14.25 ± 1.05 years; 57.5% were male, and 94.0% owned smartphones. On the IAT-20, 44.5% of adolescents had mild, 19.0% moderate, and 3.0% severe internet addiction; 66.0% were classified as smartphone-addicted on the SAS-SV. Mean daily screen time was 4.85 ± 2.15 hours, with 29.0% exceeding six hours per day. On multivariate analysis, urban residence (AOR 2.40, 95% CI 1.80–3.30), father’s education graduate and above (AOR 3.30, 95% CI 1.70–4.30), higher family income (AOR 1.90, 95% CI 1.50–3.51), smartphone use for more than three years (AOR 2.08, 95% CI 1.70–3.60), authoritarian (AOR 2.86, 95% CI 1.68–4.86) and permissive (AOR 2.34, 95% CI 1.42–3.86) parenting styles, daily screen time above six hours (AOR 4.56, 95% CI 2.68–7.76), and severe psychological distress (AOR 10.82, 95% CI 5.11–22.88) were significantly associated with internet addiction. Conclusion: Two-thirds of school-going adolescents showed some degree of internet addiction, with psychological distress, prolonged screen time, and non-authoritative parenting emerging as the strongest correlates. School-based screening and multi-level preventive interventions involving families are urgently needed.