Borderline personality disorder (BPD) is a debilitating disorder characterized by deficits in social connectedness, which is a multifaceted construct with structural (i.e., the number, diversity, or frequency of social relationships), functional (i.e., the actual or perceived resources relationships provide), and quality (i.e., the positive and negative aspects of social relationships) elements (Holt-Lunstad, 2018). However, the literature is sparse and lacks integration regarding which specific elements of social connectedness are deficient in BPD and why. This systematic review synthesized the literature on the bidirectional relationship of social connectedness and BPD. Electronic searches of three databases (i.e., PsycInfo, PsycArticles, and PubMed) identified 1,962 articles which underwent title and abstract screening and, if potentially eligible, full-text review. Sixty two articles met the eligibility criteria and underwent data extraction and risk of bias assessment. Cross-sectional research supported associations between BPD and problems in structural, functional, and quality social connectedness, with most research underscoring deficits in quality social connectedness. Preliminary longitudinal research suggested that BPD pathology predicts problems across these domains, but little to no research exists testing the reverse direction. Although people with BPD may not have difficulties forming relationships, they exhibit a range of problems within those relationships. BPD may elicit such problems in social connectedness, but it is unclear whether such issues reciprocally exacerbate and elicit BPD, and longitudinal research investigating such directionality is needed. (PsycInfo Database Record (c) 2024 APA, all rights reserved).