The experience of traumatic events in childhood is an important risk factor for the initiation and continuation of polysubstance use. This study aimed to examine the mediating role of DSM-5 level of personality functioning and maladaptive personality traits in the relationship between traumatic life events and polysubstance use. We used a mixed sample (N=290; M_age = 40; SD_age = 12.26; 75.2% males) of patients with substance use disorder (SUD; including 90 mono-drug users and 113 polysubstance users) and non-users (including 87 university students) recruited using convenience sampling method from harm reduction-oriented drug treatment centers and universities respectively in Tehran, Iran. Participants completed the Level of Personality Functioning Scale-Brief Form, the Personality Inventory for DSM 5-Brief Form, and the Life Events Checklist-Revised. The results of structural equation modeling showed that DSM-5 level of personality functioning and maladaptive personality traits partially mediated the relationship between traumatic life events and polysubstance use. Experiencing traumatic events may contribute to the risk of polysubstance use through the effect of global personality dysfunction and specific personality traits. This proposed mediational model must be replicated using a longitudinal design across different populations.