Borderline personality disorder (BPD) is one of the most studied personality disorders and is associated with significant outcomes such as suicide. Although BPD is represented in as a categorical diagnosis, it may be better characterized dimensionally, such as from the perspective of the five-factor model of general personality (FFM). The Five-Factor Borderline Inventory (FFBI) assesses BPD from the perspective of maladaptive variants of FFM traits. Previous research suggests that informant-reports may increase the validity of personality disorder assessment, providing additional information that may supplement self-report. Therefore, the current study developed an informant measure of the FFBI, Informant Five-Factor Borderline Inventory (IFFBI) and examined its convergent and discriminant validity compared with the self-report FFBI, FFM, and traditional measure of BPD. Overall, the IFFBI demonstrated good convergent validity and moderate discriminant validity with the FFBI, FFM, and other traditional measures of BPD.