Abstract The majority of young adults who use alcohol and marijuana sometimes use the two substances simultaneously. Understanding why young adults engage in simultaneous alcohol and marijuana (SAM) use may inform interventions and help offset negative consequences. To date, research has not yet examined motives for SAM use. The current study tested a 26-item measure of motives for SAM use in a community sample of young adults to identify the factor structure and to evaluate associations of subscales of SAM motives with alcohol and marijuana motives and substance use. Young adults from the Seattle metropolitan area (N = 286; 58% female, 67% White/Caucasian) were asked about their motives for using alcohol, marijuana, and SAM as well as their use of alcohol and marijuana and related consequences in the past month. Exploratory factor analysis with promax rotation identified four factors to characterize motives for SAM use: (1) conformity (8 items, a = 0.87, e.g., ìto fit in with a group I like,î ìpressure from othersî), (2) positive effects (6 items, a = 0.88, e.g., ìcross-faded effects are better,î ìto get a better highî) (3) calm/coping (3 items, a = 0.77, e.g., ìto calm me down,î ìto cope with anxietyî), and (4) social (5 items, a = 0.78, e.g., ìbecause it is customary on special occasions,î ìas a way to celebrateî). Results revealed that alcohol, marijuana, and SAM motives were moderately correlated. Even after controlling for alcohol or marijuana motives, SAM motives were associated with SAM use and marijuana use/consequences (but not alcohol use/consequences).