Background: The prevalence of overweight and obesity (OW/OB) has increased rapidly in Vietnam. This study aimed to elucidate the factors influencing OW/OB among secondary schoolchildren. Method: A survey was conducted in January 2014 in four randomly selected state schools in two Hanoi urban districts, and 821 students in grade six (11-12 years old) participated. Definitions of OW/OB followed the World Health Organization standard cut-offs. Results: Overall, 4.1% of children were underweight, 59.7% were normal weight, 17.1% were overweight, and 19.1% were obese. The odds of OW/OB were lowest among children whose parents had college/university degrees [father (aOR =0.65, 95% CI: 0.42-1.00); mother (aOR =0.63, 95% CI: 0.41-0.97)] compared with those whose parents had only a primary education. Children with an OW/OB family history had an increased risk of OW/OB. Other associated factors include parental OW/OB and birth weight (BW). The odds of OW/OB were highest among children with parents with OW/OB [father (aOR =2.022, 95% CI: 1.34-3.04); mother (aOR =2.83, 95% CI: 1.51-5.30)] compared with those with normal-weight parents. Children with both parents having OW/OB [both parents (aOR =6.59, 95% CI: 1.28-33.87) had the highest risk, followed by one parent (aOR =2.22, 95% CI: 1.50-3.27)] and then neither parent having OW/OB. Moreover, high-birth-weight children [BW ≥ 3500 g (aOR =1.52, 95% CI: 1.07-2.15)] had greater odds than did normal-birth-weight children. Children who slept 11 h per day [8-11 h (aOR =0.57, 95% CI: 0.40-0.81) or more (aOR =0.44, 95% CI: 0.22-0.87)] had lower OW/OB odds than those who slept 8 h or less. Children with specific positive lifestyle behaviours had lower risk of OW/OB than those who did not engage in positive lifestyle behaviours. The odds were lower among children who exercised for weight reduction (OR = 0.16, 95% CI: 0.11-0.23), lowered food intake (aOR = 0.12, 95% CI: 0.09-0.17), and added vegetables to their diet (aOR = 0.26, 95% CI: 0.19-0.35). Conclusion: The results suggest that parents and children with OW/OB parents or a high BW should be educated to prevent OW/OB at an early stage. Positive lifestyle behaviours should be adopted by the students.