Aims Theory‐driven, exploratory study to: (i) identify a reward drinking phenotype in young adults; (ii) evaluate this phenotype as a predictor of naltrexone response; and (iii) examine mechanisms of naltrexone in reward drinkers. Design Secondary analysis of a randomized controlled trial. Setting USA. Participants A total of 128 young adult (ages 18–25) heavy drinkers. Interventions Naltrexone versus placebo. Measurements Daily surveys assessed affect, urge, drinking, and context. The Drinking Motives Questionnaire was used to identify phenotypes based on reward (enhancement motives) and relief (coping motives) drinking. Findings We identified three profiles: “Low reward/Low relief” (14.1%; low enhancement/low coping motives); “Reward drinkers” (62.2%; high enhancement/low coping motives); and “High reward/High relief” (22.7%; high enhancement/high coping motives). Among reward drinkers (versus low profile), naltrexone significantly reduced percent days drinking to intoxication (blood alcohol concentration BAC ≥0.08) (PDI) (d = 0.56; 95% CI 0.17, 0.96) and percent high intensity drinking days (PHID) (8/10 drinks for women/men) (d = 0.32; 95% CI 0.01, 0.68). Among the high reward/high relief profile drinkers (versus low profile), naltrexone reduced PHID (d = 0.69; 95% CI 0.02, 1.50). Using profile‐informed cutoffs and observed scores (for clinical applicability): (i) among cutoff‐derived reward drinkers, we found a medium‐to‐large (d = 0.66; 95% CI 0.24, 1.16) and small effect (d = 0.28; 95% CI 0.04, 0.72) of naltrexone in reducing PDI and PHID, respectively; and (ii) among the cutoff‐derived high reward/high relief subgroup, we found a medium‐to‐large effect (d = 0.63; 95% CI 0.05, 1.1) of naltrexone in reducing PHID. Among reward drinkers (not other profiles), naltrexone reduced drinking on days a drinking event occurred by weakening the within‐day association between positive affect and urges (P < 0.05). Conclusions Naltrexone has pronounced effects in reducing risky drinking among young adult reward drinkers (high reward/low relief) by reducing urges on days when individuals have higher positive affect and are exposed to a drinking event. Naltrexone also appears to reduce risky drinking among young adult high reward/high relief drinkers, but not via the same mechanism.