Deliberate self-harm is an intentional behaviour involving direct destruction of body tissues that causes non-fatal physical trauma to the extent that bleeding or bruising occurs without conscious suicidal intent. Adolescents' self-harm is an important issue in mental health work due to its high prevalence in Thailand. This study aims to test a causal model of deliberate self-harm and identify how sex, family relationship, school connectedness, stress, resilience, and self-control influenced deliberate self-harm behaviour in Thai adolescents. A model-testing, crosssectional study was conducted to test a causal model of deliberate self-harm in Thai adolescents. Multi-stage random sampling was used to recruit 360 adolescents. Adolescents completed six selfreport instruments: the family relationship questionnaire, student-school connectedness scale, resilience factors scale for Thai adolescents, self-control questionnaire, Perceived Stress Scale-10 (Thai version), and Deliberate Self-Harm Inventory. Internal consistencies ranged from 0.81 to 0.89. Data were analysed by using descriptive statistics and structural equation modelling. The findings found sex, resilience, stress, and school connectedness had direct effects on deliberate selfharm (b = À0.139, b = À0.266, b = 0.163, and b = À0.671, respectively). Resilience and stress also mediated the links between sex (female), family relationship, school connectedness, and deliberate self-harm. The variables accounted for 65.2% of the variance in the prediction of deliberate self-harm behaviour in Thai adolescents. These findings suggest the causal model of deliberate self-harm fit the empirical data. Interventions to reduce stress and strengthen school connectedness, family relationship, and resilience among Thai adolescents should be implemented, particularly for boys, to prevent deliberate self-harm.