Objective To study the relationship between socioeconomic position (SEP) and adolescent substance use and explore the role of poor mental health in that relationship. Methods Adolescents aged 11-15 years participated in the Health Behavior in School-aged Children survey in Armenia. Robust Poisson regression and counterfactual mediation analysis were used. Results In a pooled analysis with 6512 adolescents, the adjusted prevalences of current smoking (prevalence ratio [PR] = 1.93, 95%CI = 1.29-2.88), weekly beer (PR = 1.46, 95%CI = 1.07-2.01), spirits (PR = 1.54, 95%CI = 1.01-2.37) or lifetime cannabis use (PR = 3.10, 95%CI = 0.91-10.59) were greater in low-SEP adolescents compared to the middle-SEP group. Poor mental health explained 25.6%-...