This work analysed the psychometric properties of the 6th version of the Addiction Severity Index (ASI-6) translated and adapted to the Spanish language. A multicentre, observational and prospective design was used. A total of 258 participants were included, 217 were patients (35 stable patients and 182 unstable patients), and 41 were controls. The results show satisfactory psychometric performance of the ASI-6. The degree of the internal consistency of the standardized objective scores ranged between .47 and .95. As for test-retest reliability, the values were acceptable, varying from .36 to 1. The study of the internal structure revealed a good fit to a unidimensional solution for all scales taken independently. Regarding convergent-discriminant validity, the correlations between the primary and secondary scales of the ASI-6 and the Clinic Global Impression score were low, with values from .01 to .26. Likewise, 8 of the 15 scales differentiated between controls and unstable patients. The psychometric properties of the ASI-6 Spanish version seem to be acceptable, though it is necessary to carry out new studies to test metric quality with independent samples of patients.