Buddhism is popular worldwide. It has been taught both inside and outside schools to enhance teenagers' attitudes and behaviour. To evaluate how it works, identifying teenagers' religiosity and the factors that influence it is essential. This study surveyed 140 teenagers attending retreats at three Zen monasteries belonging to the Truc Lam (Bamboo Forest) Zen sect in Vietnam, using a questionnaire to examine how the core dimensions constituting their religiosity associated with each other and with the demographics. Twelve in-depth interviews with teens were also conducted to explore how religious they were. The conclusion was that the religiosity of teenagers involved three core dimensions: religious understanding, faith and practice, which correlated with each other. Furthermore, age and gender had a considerable association with the teens' religiousness. These findings may help in expanding religious education.