In an effort to better understand factors that may explain prior findings of a positive relation between posttraumatic stress symptom severity and coping-oriented marijuana use motivation, the present study tested whether the association between posttraumatic stress symptom severity and marijuana use coping motives is mediated by difficulties in emotion regulation. Participants were 79 (39 women; M(age) = 22.29 years, SD = 6.99) community-recruited adults who reported (1) lifetime exposure to at least one posttraumatic stress disorder Criterion A traumatic event and (2) marijuana use in the past 30 days. Results indicated that difficulties in emotion regulation, as indexed by the Difficulties in Emotion Regulation Scale (Gratz & Roemer, 2004), fully mediated the association between posttraumatic stress symptom severity and marijuana use coping motives. Implications for the treatment of co-occurring posttraumatic stress and marijuana use are discussed.