A history of childhood abuse and neglect (CAN) is significantly associated with psychopathologies in adulthood, including comorbid posttraumatic stress disorder (PTSD) and substance use disorders (SUD). Difficulties in emotion regulation (ER) might influence the association between CAN and PTSD. The aim of this study was to examine the relationship between CAN and PTSD symptom severity in women with SUD and to investigate the mediating role of general difficulties in ER and its specific dimensions. Method: We examined 320 women, with a current diagnosis of at least subsyndromal PTSD and SUD, using self-report measures of CAN, PTSD symptom severity, and ER difficulties. We conducted both simple and multiple bootstrapping-enhanced mediation analysis to investigate whether general difficulties in ER and its specific dimensions mediate the relationship between CAN and PTSD symptom severity. Results: General difficulties in ER mediated the association between CAN and PTSD symptom severity. CAN significantly predicted adult PTSD symptom severity, directly and indirectly, through ER difficulties. Difficulties engaging in goal directed behavior when distressed was the only ER dimension, which mediated the effect of CAN on PTSD symptoms. Conclusions: Our results suggest that difficulties in ER and specifically difficulties engaging in goal directed behavior when distressed might constitute an influential factor in the relationship between CAN and PTSD symptom severity in a sample of SUD patients, and highlight the importance of targeting ER as a potential treatment focus for patients with comorbid PTSD and SUD.