Evidence on the relationship between alcohol consumption and body mass index (BMI) is mixed, particularly for young adults. This study explored the relationship between energy obtained from alcoholic beverages and BMI using data for 18⁻25 year-olds (n = 7691) from pooled cross-sections of the 2008⁻2014 Health Survey for England and the Scottish Health Survey. Energy obtained from alcoholic beverages (excluding mixers) on the heaviest drinking day in the past week was expressed as percentage of total recommended dietary allowance (RDA) of energy (% RDA Energy). Linear regressions were estimated of BMI on alcohol intake categories controlling for intake frequency, physical activity, longstanding illness and other covariates, with separate analyses for men and women, and by beverage type. Significant associations with BMI were observed with the 'Very High' category of alcohol intake (>75% RDA Energy) for men (p < 0.001, 1.74, 95% confidence interval (CI) 0.98, 2.49) and with the "High" (>50% to 75% RDA Energy) (p < 0.001, 1.67, 95% CI 0.26, 2.58) and above category for women, when compared with the Low (>0⁻25% RDA Energy) category. Young adults drinking the highest levels of alcohol on a single occasion were more likely to be obese than those with the lowest intake. Interventions to address internationally rising youth obesity rates should also consider reducing alcohol consumption by increasing alcohol prices, and reducing availability and marketing exposure.