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Twenty-first birthdays are associated with extreme levels of heavy drinking and alcohol-related harm. Effective preventive interventions that are acceptable to young adults are needed. The current study tested the efficacy of a brief text-message intervention for reducing 21st birthday alcohol involvement designed to correct perceived 21st birthday drinking norms and provide protective behavioral strategies (PBS). We also examined potential moderators and mediators. College students (n = 200) with an upcoming 21st birthday completed a baseline assessment and were randomized to a text-message intervention or an assessment-only control condition. For participants in the intervention group, Message 1 (sent one day before the birthday celebration) focused on personalized normative feedback, and Message 2 (sent day of the birthday celebration) discussed PBS to minimize risk. Primary outcomes were assessed using responses to a follow-up assessment the day after their birthday celebration (93% completion rate). Zero-inflated negative binomial regression analyses did not reveal an overall intervention effect for estimated Blood Alcohol Content (eBAC) or alcohol problems on the 21st birthday celebration. In partial support of our hypothesis, there was an indirect effect of perceived 21st birthday norms on 21st birthday eBAC. The intervention was associated with reduced perceived norms, which was, in turn, related to a lower eBAC. There was a 3-way interaction between drinks per week, anticipated eBAC, and intervention condition for the count portion of actual eBAC such that the intervention reduced eBAC among a high-risk subset of the sample. Future research may benefit from further refining the personalized normative feedback (PNF) component of 21st birthday interventions.