Background and aims: Experience of alcohol-induced memory blackouts in adolescence may be an important risk factor for later harms. This longitudinal study (i) modelled trajectories of alcohol-related blackouts throughout adolescence, (ii) explored early-adolescent predictors of blackout trajectories and (iii) examined the association between blackout trajectories and alcohol use disorder (AUD) symptoms. Design: Longitudinal study in which data from six annual surveys of a longitudinal cohort of Australian adolescents were used to model latent class growth trajectories of blackouts, adjusting for alcohol consumption frequency and typical quantity. Regression models were used to determine whether parent, child and peer factors at baseline (mean age = 12.9) predicted profiles of blackout trajectory membership and whether blackout trajectories predicted meeting criteria for AUD in early adulthood (mean age = 19.8). Setting and participants: Australian adolescents (n = 1821; mean age = 13.9–18.8 years). Measurements: Alcohol-related blackouts, alcohol consumption frequency, typical consumption quantity and DSM-5 AUD in early adulthood were all self-reported. Findings: We identified a three-class solution: delayed alcohol initiation, rare blackouts (n = 701; 38.5%); early initiation, rare blackouts (n = 869; 47.7%); and early initiation, increasing blackouts (n = 251; 13.8%). Female sex was associated with increased risk of early initiation, increasing blackouts relative to delayed initiation, rare blackouts [relative risk ratio (RRR) = 3.90; 99.5% confidence interval (CI) = 1.96, 7.76] and relative to early initiation, rare blackouts (RRR = 2.89; 99.5% CI = 1.42, 5.87). Early initiation, rare blackouts [odds ratio (OR) = 1.96; 99.5% CI = 1.17, 3.29] and early initiation, increasing blackouts (OR = 4.93; 99.5% CI = 2.32, 10.48) were each associated with increased odds of meeting criteria for AUD in early adulthood relative to delayed initiation, rare blackouts. Early initiation, increasing blackouts was associated with increased odds of meeting criteria for AUD in early adulthood relative to early initiation, rare blackouts (OR = 2.51; 99.5% CI = 1.18, 5.38). Conclusions: Females in Australia appear to be at higher risk of adolescent alcohol-related blackouts independent of alcohol consumption levels and age of initiation. Alcohol-related blackouts may be associated with later alcohol use disorder. © 2021 Society for the Study of Addiction