Co-administration of tobacco/nicotine and marijuana (e.g., using both products on the same occasion by mixing them in the same delivery device) is a risky, yet common, form of co-use among young adults. Understanding motivations for co-administration co-use, and how these are associated with subsequent use and related problems, is needed to inform policy, prevention, and intervention efforts. We conducted a latent class analysis on 342 young adults with past-year co-administration of tobacco/nicotine and marijuana to determine how emergent classes of 16 co-use motives were associated with use and co-administration co-use of tobacco/nicotine and marijuana one year later. Four classes emerged: (1) a high all class that reported high endorsement of all motives, (2) a high coping class that reported high endorsement of co-use due to coping with stress and bad moods, (3) a high social/physiological reinforcement class that reported low endorsement of coping but high probability of social and relaxation motives and motives to even out effects of each substance, and (4) a low all class that reported low endorsement of all motives. The high all and high coping classes reported greater frequency and quantity of single substance use and co-administration. The high coping class reported more marijuana consequences than the low all class. The high all and high coping classes reported younger age of initiation of each individual substance and co-administration. Considering co-use motives may be useful in identifying those at highest risk for future use and consequences and tailoring interventions to the distinct needs of co-use subgroups. (PsycInfo Database Record (c) 2021 APA, all rights reserved).