Purpose Obesity is one of the major factors implicated in raising morbidity and mortality in adults as well as adolescents and children. Obesity is increasing worldwide and there is evidence that this increase has been faster among the developing countries. Thus, the need to determine the pattern and determinants of overweight and obesity among adolescents. Methods This was a cross-sectional study involving 400 adolescents aged 14.38 ± 2.11years from randomly selected secondary schools. Questionnaire, weight and height metre was used to collect data. Descriptive statistics of mean ± standard deviation, frequency counts and percentage were used to summarize participants’ variables. Inferential statistics of Pearson’s correlation, independent samples t-test, Chi-square and binary logistic regression were used in hypotheses testing with 0.05 alpha levels. Result Mean of Body mass index(BMI) and daily physical activity(PA) in the last one week were 23.14 ± 3.49kg/m2 and 1.84 ± 0.37 respectively with a point prevalence of 0.2(20 per 100 adolescents). There was a significant difference in BMI pattern and PA (with t-value − 3.94 and 5.31) between adolescents from public and private schools. Activity of daily living related-PA was found to have a significant association with BMI (P-value = 0.001, r = -0.17) and patterns of overweight and obesity among secondary school students (χ2 = 44.61, p < 0.001). Four predictors (PA during school recreation period, weight, height, and School type), together classified students who were likely to be overweight/obese with variance Nagelkerke R2 = 93.3%. The total prediction success is 91.8%, with 99.3% and 92.6% of those not overweight/obese and those that were correctly classified respectively. Conclusion This study highlights PA during school recreation period, weight, height, and School types as determinants of overweight/obesity in adolescents and also high prevalence of overweight and obesity.