Background: Adolescent alcohol consumption has fallen in most Western countries over the past two decades, while immigrants and children of immigrants from low-consumption countries constitute a growing proportion of teenagers in many Western nations. We investigate the extent to which immigrants and children of immigrants have contributed to the decline in adolescent heavy episodic drinking in Oslo, the capital of Norway.Methods: We use repeated cross-sectional survey data on adolescents in grades 9-11 in Oslo (aged around 14–16, N = 54,474) from 1996 to 2018. We use data on heavy episodic drinking/intoxication in the past 12 months (dichotomized), immigrant background, sex and grade. We decompose the trend into components attributable to changes in the demographic composition of the adolescent population (by immigrant background, grade and sex), and to changes in drinking patterns within different groups. Confidence intervals (CIs) are obtained by bootstrap resampling.Results: The proportion of adolescents with immigrant backgrounds increased from 21% to 35% over the time span. The proportion reporting having been intoxicated fell from 42% to 25%. Most of the decline stems from reduced heavy episodic drinking in the majority population, accounting for 70.8% of the reduction (95% CI: 67.5–74.2). The increased proportion of adolescents with an immigrant background accounts for 21.4% of the decline (95% CI: 19.2–23.8).Conclusions: An increasing proportion of immigrants and children of immigrants with low alcohol consumption explains one-fifth of the decline in the prevalence of adolescent heavy episodic drinking in Oslo.