•Brief daily mindfulness practice increases trait mindfulness and self-compassion.•Relaxation practice improves emotion regulation in a high BPD trait sample.•Both practices do not differ in their effects on any of the outcome variables. Borderline personality disorder (BPD) is a psychological disorder characterized by dysregulation across multiple domains. While selected mindfulness-based interventions are effective in addressing symptoms of BPD, less is known regarding the effects of engaging in brief mindfulness practices on affective correlates of BPD. The present study investigated the effects of engaging in brief, daily mindfulness practice in a sample of young adults with elevated BPD symptoms. Ninety-two participants were recruited and randomly assigned to 2 weeks of daily mindfulness meditation or relaxation practice (active control), or to a no-practice control condition. Participants completed measures assessing depression, anxiety, stress, shame, difficulties with emotion regulation, trait mindfulness, and self-compassion before and after the practice period. Compared to the no-practice control condition, mindfulness practice resulted in significant improvements in trait mindfulness and self-compassion, whereas relaxation practice led to reductions in emotion regulation difficulties. No significant differences were found on any of the other outcomes between the mindfulness and relaxation conditions. The findings suggest that while both mindfulness and relaxation-based practices are effective in targeting selected transdiagnostic processes associated with BPD, more intensive interventions may be required for the practices to induce changes at the level of psychological symptoms.