Background: The Borderline Symptom List-23 (BSL-23) is a reliable and valid self-report instrument for assessing Borderline Personality Disorder (BPD) severity. The psychometric properties of the original version have proven to be adequate. The aim of the present study was to validate the Spanish language version of the BSL-23. Methods: The BSL-23 was administered to 240 subjects with BPD diagnosis. Factor structure, reliability, test-retest stability, convergent validity, and sensitivity to change were analyzed. Results: The Spanish version of the BSL-23 replicates the one-factor structure of the original version. The scale has high reliability (Cronbach's alpha=.949), as well as good test-retest stability, which was checked in a subsample (n=74; r=.734; p<.01). The Spanish BSL-23 shows moderate to high correlations with depressive symptomatology, state and trait anxiety, hostility and impulsivity scores and BPD measures. The Spanish BSL-23 is able to discriminate among different levels of BPD severity and shows satisfactory sensitivity to change after treatment, which was verified by assessing change before and after 12 group sessions of Dialectical Behavioral Therapy in a subgroup of 31 subjects. Conclusions: Similar to the original BSL-23, the Spanish BSL-23 is a reliable and valid instrument for assessing BPD severity and sensitivity to change.