Background: Substance use disorder (SUD) is associated with psychological, biological, and social problems. To date, existing treatments have a high dropout rate. Objective: This study aimed to investigate the psychosocial factors related to dropout from prolonged inpatient treatment due to SUD. Methods: A nine-week longitudinal follow-up study of 148 adult men inpatient for treatment for SUD. Psychosocial measures, aspects related to treatment, substance use, and sociodemographic data were collected using self-completed questionnaires. The main outcome was treatment dropout nine weeks after initial contact. Binomial logistic regression was used to evaluate the association of variables with dropout. Results: Prolonged inpatient treatment dropout was related to anxiety, depression, stress, self-judgment, expression of anger, positive and negative affect, negative urgency, lack of perseverance, and impulsivity, but not to sociodemographic characteristics or characteristics related to the substances consumed and the history of use. Conclusion: Psychological factors, such as negative emotional states and impulsivity, were the main predictors of dropout. These results highlight the relevance of emotional management skills to a male population in preventing the treatment dropout associated with SUD.