Alexithymia is related to a higher severity of substance use disorders (SUD); however, few longitudinal studies have been performed on how alexithymia impacts treatment outcomes. This study aims to evaluate alexithymia as a factor that could influence retention and the time of the first relapse in a one-year follow-up in an outpatient treatment center for SUD. In total, 126 SUD outpatients (74.60% males; M age = 43.71, SD = 14.61 years) were evaluated at baseline with an AdHoc questionnaire for sociodemographic variables, the European Addiction Severity Index (EuropASI), the Semi-structured Clinical Interview for Axis I and Axis II Disorders of the DSM-IV, and the Toronto Alexithymia Scale (TAS-20). The prevalence of alexithymia was 41.3% and the mean score of TAS-20, was 57.27 (SD = 12.84). At baseline, alexithymia was related to a lower education level, cannabis use disorder, the psychological item of EuropASI, and mood spectrum disorders. In the Kaplan-Meier analysis, SUD patients with alexithymia were in treatment for less time and presented earlier relapses than non-alexithymic patients. In the Cox regression, alexithymia was only associated with less time in treatment. Therefore, alexithymia may have an important role in the outcomes of SUD treatment, and hence, therapeutic approaches for SUD that cover emotional impairments associated with alexithymia should be investigated and developed.Copyright © 2020 Elsevier Ltd. All rights reserved.