In adolescent best friendship dyads, we examined: (a) similarity in substance use and decision‐making; (b) associations between participants' decision‐making and their own and best friend's substance use, (c) the influence of relative popularity within the dyad on these associations. Participants (n = 172; 12–18 years) named their best friend, completed popularity ratings, and a substance use questionnaire. Computer tasks were administered to assess risk‐taking and immediate reward preferences. Reciprocated same‐sex best friendship dyads (n = 49) were distinguished on their popularity, and we controlled for age differences between dyads in the analyses. Best friends were similar in substance use and risk‐taking preferences. More popular friends' risk‐taking preferences were positively associated with alcohol use of less popular friends. These findings underscore best friendship similarity in risky behaviors, and the influence of popular friends.