Obesity in children is rapidly becoming a major public health problem in Hong Kong; the prevalence of overweight in children increased from 16.2% to 22.2% between 1998 and 2008. Healthy eating programs and policies exist, but only for primary schools, and students' eating habits change when they are exposed to the new environment in secondary schools. Therefore, this study examined the relationship between available food in secondary school tuck shops and students' purchasing preferences. A cross‐sectional survey was conducted from mid‐March to mid‐April 2016 across six secondary schools, using a questionnaire to measure students' healthy eating knowledge and eating habits and a checklist to explore food options in each school's tuck shop. Linear regression was used to explore the relationship between eating knowledge and eating habits and logistic regression to explore the relationship between available foods at the tuck shops and the children's purchasing preferences. A total of 374 junior secondary students participated in the survey. A weak, positive correlation was found between healthy eating knowledge and healthy eating habits. No association was found between available food at tuck shops and children's purchasing preferences, although 81% of the students reported that they would choose healthy food at the tuck shops if offered. The results reveal a definite knowledge‐attitude gap where healthy eating is concerned. Despite there being no significant association between the food offerings in tuck shops and students' measured purchasing preferences (p > 0.05), most students stated that they would choose healthy food if offered, illustrating the urgent need to assess the variety and nutritional quality of foods available in secondary school tuck shops. This article is protected by copyright. All rights reserved.