Gambling disorder is a high comorbid disorder in substance abusers which conjunct appearance is related to worse symptomatology and evolution. Nevertheless, the research on the risk factors that may explain this comorbidity is scarce. We build of the self-regulation theory of attachment and addiction to examine if insecure attachment is related to gambling comorbidity in substance abuse disorder and the mediating role of alexithymia in this process. A cross-sectional study was carried out with 369 clinical patients with substance use disorder of which 69 presented comorbid gambling disorder diagnosed with the DSM-5 criteria. Results showed that insecure attachment was more prevalent in the group with comorbid gambling. In this group, the alexithymia levels were also higher and mediated the relationship in between attachment and gambling disorder comorbidity even controlling for several sociodemographic variables. This research indicates that insecure attachment enhances the risk of gambling comorbidity on substance abusers due to the detrimental effect on the self-regulation of emotion. Thus, interventions directed to increase the identification, expression and awareness of emotions might help to reduce comorbidity of gambling of substance use disorders.Supplementary InformationThe online version contains supplementary material available at 10.1007/s10899-022-10154-2.