Simultaneous alcohol and marijuana (SAM) use is reported to be associated with heavier alcohol and marijuana use and more negative consequences, but less is known about the social, physical, and temporal contexts of SAM use. Young adults ( = 326, 51.2% male, 49.1% White non-Hispanic) who reported past-month SAM use completed up to 14 daily surveys across five bursts that assessed SAM use and negative consequences and social, physical, and temporal contexts. We used multilevel models to examine SAM use contexts' associations with alcohol/marijuana quantity and consequences. The social context of alone only (vs. with others only) was associated with consuming fewer drinks. Physical contexts that included using both at home and outside the home (vs. only at home) were associated with greater alcohol and marijuana quantity and negative consequences (but not after controlling for alcohol quantity); use outside the home only (vs. only at home) was associated with more alcohol use, more alcohol consequences (but not after controlling for alcohol quantity), and fewer marijuana consequences (even after controlling for marijuana quantity). The temporal context of first engaging in SAM use before 6 P.M. (vs. after 9 P.M.) was associated with greater alcohol and marijuana quantity and more marijuana consequences (but not after controlling for number of hours high), and first engaging in SAM use between 6 and 9 P.M. was associated with more hours high. SAM use contexts such as using with others, using outside the home, and using earlier in the evening are typically associated with greater alcohol/marijuana quantity and consequences.