Previous research has repeatedly demonstrated positive associations between negative affect (NA) and borderline personality disorder (BPD) symptoms in daily life, but studies have rarely addressed potential effects of positive affect (PA). Consequently, little is known about how PA in daily life covaries with symptoms of BPD. The authors assessed the effects of both PA and NA levels on BPD symptom severity in a sample of 81 treatment-seeking women diagnosed with BPD over a period of 21 days, employing a daily diary design. Using multivariate multilevel modeling, the authors obtained negative associations between PA levels and daily BPD severity in total and at the level of the individual symptoms inappropriate anger, affective instability, emptiness, identity disturbance, and paranoid ideation/ dissociation. Moreover, the authors replicated previously reported positive associations between NA and BPD severity for all nine symptoms. Future research can address whether increasing PA in the treatment of BPD may potentially help reduce symptom burden.