Unhealthy diets were found to be the main contributor to the overweight problem among adolescents. In this study, we aim to identify the factors causing unhealthy eating habits in adolescents. School-aged children and adolescents participated in this cross-sectional observational study with additional school and parental consent. A self-administered survey was conducted by 30 primary schools and 25 secondary schools. Participants were asked about the frequency of consuming unhealthy food and the types of unhealthy food consumed. A descriptive analysis was performed to demonstrate the proportions of characteristics. The prevalence of the outcome among participants of various factors was also analyzed using separate binary regression models. A total of 4884 responses were collected. Among primary school students (grade 4, mean age: 10.06), people who (1) were actively gaining weight (aOR: 1.651, 95% CI 1.006-2.708, p = 0.047), (2) went to bed after 11 p.m. (aOR: 1.652, 95% CI 1.065-2.563, p = 0.025), (3) had more than 2 h of gaming (aOR: 2.833, 95% CI 1.913-4.195, p < 0.001), (4) suffered from self-report depressive symptoms (aOR: 1.753, 95% CI 1.233-2.493, p = 0.002) was more likely to consume unhealthy food. As for secondary school students (grade 3, mean age: 15.28), (1) males (aOR: 1.266, 95% CI 1.0004-1.601, p = 0.0496), (2) average-to-high socioeconomic status (Average: aOR: 1.471, 95% CI 1.115-1.941, p = 0.006; High: aOR: 2.253, 95% CI 1.585-3.202. p < 0.001), (3) having more than 2 h of gaming (aOR: 1.342, 95% CI 1.069-1.685, p = 0.011), (4) suffering from psychological distress (aOR: 1.395, 95% CI 1.051-1.852, p = 0.021) were associated with the increased odds of consuming unhealthy food. Several lifestyle and health factors were significantly associated with unhealthy eating behaviors in school-aged children and adolescents in Hong Kong, sharing similarities with many other countries. In conjunction with implementing a policy that addresses factors for unhealthy eating habits, further research should investigate potential interventions targeting these factors to ultimately tackle the overweight and obesity concern for children and adolescents in Hong Kong.