Background: Loneliness is commonly experienced during adolescence and has been associated with negative health outcomes. Objectives: The present study aimed at assessing the factors associated with loneliness and investigating the relationship between loneliness and health risk behaviors among adolescents in the association of southeast asian nations (ASEAN). Methods: This cross-sectional study included 30 284 schoolgoing adolescents (aged 13-15 years, mean age = 14. 1 years, SD = 0. 8) from 7 ASEAN countries, who took part in the global school-based student health survey (GSHS) between 2007 and 2013. The measure inquired about loneliness, health risk behaviours, and protective factors. Results: Across the 7 ASEAN countries, 7. 8% of the adolescents reported mostly or always being lonely and 31. 3% reported sometimes being lonely in the past 12 months. In multivariable logistic regression, female gender, older age, living in a low or lower income country, not having close friends, bullying victimization, and lack of parental or guardian support were associated with mostly or always feeling lonely in the past 12 months. After adjusting for sociodemographic and social supportive factors, it was found that loneliness was associated with poor mental health, substance use, aggression, and other health risk behaviours. Conclusions: A significant proportion of adolescents were experiencing loneliness, which was found to be associated with various health risk behaviours. Thus, it is important to recognize lonely adolescents early in to prevent more serious poor mental health and other health risk behaviours.