Background: The prevalence of obesity is increasing worldwide, both in developed and developing countries. In Nigeria, obesity is emerging as an important public health problem. Childhood and adolescent obesity results in adult obesity with the resultant morbidities. Objectives: This study determines the prevalence of overweight and obesity in apparently healthy secondary school adolescents. Methods: This study was a cross-sectional study of secondary school adolescents aged 10-19 years randomly selected from two public schools and three private schools. The weights and heights of the study subjects were measured using standard equipment. BMI was computed using the standard formula weight (kg) / height2 (m). Using the WHO; 2007 age and sex-specific BMI percentile cut-offs, the subjects were classified as underweight (3rd to <15th percentile), normal (15th to <85th percentile), overweight (85th to <97th percentile) or obese ≥97th percentile. Results: Data were initially collected from 1250 participants, but 52 were excluded from improperly completed questionnaires. Thus, 1198 students were ultimately included in the study, giving a response rate of 95.8%. These included 621 females (51.8%) and 577 males (48.2%) aged 10-19 years, giving an F: M ratio of approximately 1: 0.9. The mean age of the students was 15.07 ±1.96 years overall, 15.13 ± 2.08 years for males and 15.03 ± 1.83 years for females. There was no statistically significant difference between males and females in the distribution of age groups (p=0.12). The mean BMI was 21.51±3.57kg/m2 for females and 20.22±3.16kg/m2 for males. The BMI was significantly higher in females in all age groups (p<0.001) except for those aged 10-<12 years (p=0.13). The prevalence rates of overweight and obesity were 14.4% and 5.1%, respectively. The prevalence rates of overweight and obesity were significantly higher in females than males (17.7% vs 10.7%, 5.6% vs 4.5% respectively (p<0.001) and most prevalent among the early adolescence (10<12yrs; p= 0.04). Conclusion:  Prevalence of overweight and obesity is high among secondary school adolescents in Onitsha, Anambra state. There is a need for regular monitoring of weight and height as an early measure to prevent and control overweight and obesity.