Background: Bipolar disorder (BD) and borderline personality disorder (BPD) are both serious psychiatric conditions that elevate the risk for harmful outcomes. Although these conditions represent distinct diagnostic entities, existing research suggests that approximately 20% of individuals with BD meet the criteria for comorbid BPD. Objectives: The main aim of this study was to assess the effect of comorbid BPD on the cognitive functions of patients with BD. Methods: This cross-sectional study was conducted at the Department of Psychiatry Zagazig University Hospital on 150 patients with BD, out of them there were 40 cases have comorbid BPD. All study population subjected to full history taking, Structured Clinical Interview for DSM-IV Axis I, Axis II Disorders (SCID-I,SCID-II), and Montreal Cognitive Assessment (MoCA). Results: There was statistically significant difference between the two groups regarding age, sex, education, occupation, marital status, and smoking. Respecting the psychiatric and family history, there was statistically significant difference between the two groups. There was statistically significant difference between the two groups regarding clinical data MOCA score and cognitive assessment by MOCA score. By comparing between males and females regarding MOCA score, the results was statistically significant. While comparing between cases who were on regular treatment (TTT) and irregular TTT regarding MOCA score, the results was statistically significant. Conclusion: In conclusion; the current study showed that the presence of comorbid BPD was associated with worse cognitive functioning in BD patients and more severe cognitive impairment.