Background: Social stigma is the most common and challenging burden of care on the family of people with Borderline Personality Disorder (BPD) In Iran, despite the cultural and social influences, this issue has been less studied. Therefore, present study was conducted to determine the lived experiences of caregivers of patients with BPD of social stigma. Materials and Methods: This qualitative study was performed at Ibn Sina hospital in Mashhad, Iran from 2017 to 2019. Participants were selected by purposive and snowball sampling method. Data were collected through semi-structured interviews. Data saturation was achieved after 16 interviews. Finally, the data were analyzed by the method proposed by Diekelmann (1989). Results: In data analysis, one main theme and two sub-themes emerged. The main themes include Black shadow. Two sub-themes consisted of society dagger and secrecy. The sub-theme of society dagger included the two common meanings (inner turmoil in response to the stigma of others and weakening of family status among relatives and acquaintances). The sub-theme of secrecy comprised of the three common meanings (concealment of disease, hide hospitalization, and seclusion). Conclusions: An understanding of the experience of family stigma can lead to the development of supportive strategies to manage this problem among caregivers of patients with BPD. Nurses can support caregivers by offering them opportunities to discuss how stigma is disrupting their caregiving roles. They can also support the caregivers in negotiating the experienced social and emotional distress and when necessary, refer them to the other members of healthcare teams.