Recent research suggests that deficits in the ability to be mindful may be related to core aspects of borderline personality disorder (BPD). Mindfulness plays a central role in BPD treatment, and evidence also indicates that mindfulness is the most commonly practiced of the skills taught in dialectical behavior therapy (DBT). The present study investigated whether a 10-week mindfulness training program would improve BPD symptoms and mindfulness-related capacities in a sample of individuals diagnosed with BPD. A total of 64 participants (mean age = 31.64, SD = 6.9; 86 % female) were randomized to 10 weeks of mindfulness (n = 32) or interpersonal effectiveness skills training (control group; n = 32). BPD symptoms and mindfulness capacities were measured at pre- and post-intervention. Compared to the control group, participants assigned to mindfulness experienced a significantly greater reduction and increase, respectively, in BPD symptoms and decentering capacity. Treatment response rates (in reference to BPD symptoms) were higher for the mindfulness group (40 vs. 13 %). Interpersonal effectiveness alone did not result in improvements on any outcome measures. These findings suggest that mindfulness training may be a useful approach to decreasing BPD symptoms while simultaneously improving mindfulness capacities.