The study was purposed to assess the risk and determinant factors of obesity among selected Ghanaian senior high school adolescents. A cross-sectional study was used to establish the key determinants of high levels of obesity among Ghanaian adolescents. Under the umbrella of the positivist paradigm, the research was founded on a systematic quantitative approach to gathering and analysing data from 496 participants of seven purposively sampled senior high schools in the Greater Kumasi Metropolis. The questionnaire was employed to obtain demographic information and assess adolescent risk factors related to obesity. The determinants of obesity relevant to adolescents included dietary practices, sedentary behaviour and physical activity. The regression analysis indicated significantly unhealthy food consumption (β = 0.169, p < .001) and sedentary lifestyle (β = 0.148, p = .024) predicted obesity. Physical activity fell short of significance (β = -0.013, p = .886). The model overall accounted for 37.2% of the variance in adolescent obesity (R² = 0.372). The findings showed that a majority of the respondents were aged 16–20 years with a reasonably equitable gender split. Not taking enough fruits and vegetables, skipping breakfast, and spending too much time on screens were major unhealthy behaviours leading to obesity among final-year students. The respondents’ believed obesity was a long-term disease, but they were unclear about the social and health-related impact. The results illustrated the multiple causes of adolescent obesity and emphasised the necessity for a comprehensive multisectoral approach, including schools, parents and local policymakers to address this phenomenon.