Borderline personality disorder (BPD) is characterized by a pervasive pattern of instability of interpersonal relationships, self-image, and affects, and marked impulsivity beginning by early adulthood and present in a variety of contexts. Researchers have proposed that BPD may be associated with neurocognitive dysfunction. However, the diagnosis of BPD in patients with intellectual disability, where developmental brain abnormality is inherent, has rarely been reported in the literature. Furthermore, it is uncommon to see the diagnosis of BPD made in individuals with intellectual disability in clinical practice. The cases of three patients diagnosed with BPD in the context of intellectual disability are presented. Challenges in making the diagnosis of BPD in this population are discussed, as is the importance of making an accurate diagnosis for guiding effective treatment.