BACKGROUND: Given the advent of the internet and its growing resources as well as the availability of smart mobile phones among the younger generation, Problematic Internet Use (PIU) has become a globally identified contemporary issue of public health concern. This study aimed to determine the prevalence, self-reported effects, and associated factors of problematic internet use among in-school adolescents in Calabar Municipality, Nigeria. METHODS: A descriptive cross-sectional study design and a multistage sampling technique were used to select 422 respondents systematically. The 18-item Problematic and Risky Internet Use Screening Scale (PRIUSS) was adopted as instrument. Data analysis was conducted using SPSS version 23. Descriptive statistics and inferential analysis (chi-square tests and multivariate logistic regression models) were applied to measure associations and effects." RESULTS: The findings in this study revealed a 62.6% prevalence of PIU. Chi-square tests showed significant associations between PIU and female gender, caregiver's low education, living with siblings, parental unawareness, loneliness, idleness, isolation, low self-esteem, poor physical relationships (p ≤ 0.027). Multivariate analysis found lower odds of PIU among adolescents aged 15-19 (aOR = 0.236, p < 0.001), females (aOR = 0.557, p = 0.009), and those with primary-educated caregivers (aOR = 0.273, p = 0.008). CONCLUSION: A multi-disciplinary approach is essential to regulate adolescent internet use in Nigeria, requiring coordinated efforts across individual, community, institutional, and policy levels, alongside promoting digital literacy and responsible online behaviour.