OBJECTIVE: Despite protective behavioral strategies (PBS) being an important part of alcohol-prevention programs, use of PBS is suboptimal, and research is needed to determine factors associated with the use and non-use of PBS. The present study examined daily-level associations between situational familiarity (i.e., familiarity with locations and people) and the use of alcohol-related PBS among adolescents and young adults. METHOD: Participants (analysis RESULTS: Within-person results indicated that when participants had elevated (i.e., higher than their own average) familiarity with their location, they were less likely to use serious harm-reduction PBS (rate ratio [RR] = 0.94, CONCLUSIONS: The study suggests that PBS use, particularly for serious harm-reduction and stopping/limiting strategies, varies among adolescents and young adults based on familiarity with location and people. Alcohol-prevention interventions, including just-in-time interventions, should consider how to promote PBS use, particularly in familiar locations and with less familiar people.