ABSTRACT Adolescent loneliness is prevalent and has serious adverse impacts on mental and physical health outcomes. This study aimed at examining factors associated with loneliness among school-going adolescents. The country of interest is a low-income country, Timor-Leste, where empirical studies are lacking. Data from the Global School-Based Student Health Survey Timor-Leste 2015 (n = 3455) were used for secondary analyses. An ordered probit model was utilized to assess demographic, lifestyle, social, and parental factors correlated with different levels of loneliness (no loneliness, moderate loneliness, and severe loneliness). Approximately 46.3% of adolescents felt moderately lonely and about 13.5% felt severely lonely. Older adolescents were more likely to experience severe and moderate loneliness than younger adolescents. Going hungry and being bullied were associated with an increased likelihood of being severely and moderately lonely. Adolescents had a higher probability of experiencing moderate and severe loneliness if they had sleep difficulties and were physically active. Having understanding parents was positively associated with the likelihood of severe loneliness. In conclusion, a large proportion of adolescents in Timor-Leste have feelings of loneliness, which have been found to be associated with demographic, lifestyle, social, and parental factors. Policymakers should recognize the correlated factors of loneliness in order to formulate a more effective intervention strategy.