Borderline personality disorder (BPD) and mood disorder (MD) can be difficult to differentiate from each other due to several overlapping clinical features. Among BPD symptoms, chronic dysphoria can be mistaken for major depression, while affective instability may be confused with the depressed and elevated mood episodes of bipolar disorder (BD). Conversely, in those with BPD, co-occurring MDs can be difficult to rigorously assess and treat. Even though there is moderate to high co-occurrence between these conditions, BPD and MDs have distinct facets of impulsivity, affective instability, and mood symptoms. Furthermore, BPD, MD, and their co-occurrence predict courses of illness, prognosis, treatment outcomes, and suicide risk. Consequently, thorough assessment and differential diagnosis of these conditions should inform treatment planning and clinical management in both BPD and MD.