AbstractIntroductionIn the 21st century, there has been a decline in alcohol use among adolescents in most Nordic countries, while trends of cannabis use have diverged. We explore how alcohol and cannabis use, respectively, and co‐use of the two substances, have changed among Nordic adolescents. Three hypotheses are used to frame the study: (i) cannabis use has substituted alcohol use; (ii) there has been a parallel decline in both substances; and/or (iii) there has been a ‘hardening’ of users, implying that alcohol users increasingly use cannabis.MethodsData from the European School Survey Project on Alcohol and Other Drugs, conducted among 15‐ to 16‐year‐olds in Denmark, Finland, Iceland, Norway and Sweden (N = 74,700, 49% boys), were used to explore trends of past‐year alcohol and cannabis use in the period 2003–2019.ResultsThe proportion of adolescents reporting alcohol use decreased significantly in all Nordic countries except Denmark. The proportion of those using cannabis only was low (0.0%–0.7%) and stable in all countries. The total number of substance use occasions declined among all adolescents in all countries but Denmark. Among alcohol users, cannabis use became increasingly prevalent in all countries but Denmark.Discussion and ConclusionsWe found no support for the ‘parallel decline hypothesis' in alcohol and cannabis use among Nordic adolescents. Partially in line with the ‘substitution hypothesis’, cannabis use accounted for an increasing proportion of all substance use occasions. Our results suggests that the co‐use of alcohol and cannabis has become more common, thus also providing support to the ‘hardening’ hypothesis.