Objectives: Previous research work suggests a positive association between secure attachment and increased therapy adherence (TA) in different patient groups. However, there is still a strong need for research focusing on the influence of attachment on TA in substance use disorder (SUD) treatment. Hence, this study attempts to investigate the predictive value of different attachment patterns concerning TA in SUD inpatients. Results: 122 (34 female) SUD inpatients completed the Attachment Style Questionnaire (ASQ) during the entry phase of therapeutic community treatment. After 6 weeks, subjects who remained in therapy (n = 47) completed the ASQ for a second time. In line with the literature, agglomerative Cluster Analysis suggested a two-cluster solution (Cluster I: increased secure attachment pattern; Cluster II: increased insecure attachment pattern). Notably, inpatients in Cluster I were more likely to drop out of treatment within the first 6 weeks (p < .001). Furthermore, subjects showed less “Confidence in Self and Others” (p < .05) after 6 weeks of treatment. Our findings indicate a negative predictive value of increased attachment security for TA in SUD inpatients. This finding probably mirrors a more realistic kind of self-assessment. More generally, the importance of considering attachment styles in SUD treatment is underlined.