OBJECTIVE: Simultaneous cannabis and alcohol use is common, but few studies have examined normative perceptions of simultaneous use. This study examined unique associations of baseline descriptive norms for simultaneous use (i.e., perceptions about others' simultaneous use) with simultaneous use behaviors assessed via daily surveys. METHOD: Young adults reporting frequent simultaneous use ( RESULTS: Multilevel models revealed that, controlling for descriptive norms for cannabis and alcohol use in general, perceiving more frequent simultaneous use among friends (but not peers) was significantly associated with a greater tendency to engage in simultaneous use relative to cannabis-only use across days. Further, perceiving heavier cannabis and alcohol consumption during simultaneous use occasions among friends (but not peers) was significantly associated with greater quantities of cannabis and alcohol consumed, respectively, across simultaneous use days. CONCLUSIONS: Descriptive norms for simultaneous use contribute uniquely to simultaneous use behavior, over and above norms for cannabis use and alcohol use in general. Findings may inform norms-based interventions for young adults who engage in simultaneous use. (PsycInfo Database Record (c) 2025 APA, all rights reserved).