Posttraumatic stress disorder (PTSD) is highly comorbid with personality disorder (PD). We evaluated whether comorbid PTSD has a negative effect on patients’ ability to benefit from evidence‐based psychological treatments for PD and tested potential mediators of any effect. The effect of PTSD on self‐harm and borderline PD symptoms was evaluated in 90 patients with borderline PD who were receiving dialectical behavior therapy or mentalization‐based therapy. Self‐report and interview measures were administered every 3 months over a 12‐month period. Three‐quarters of participants met criteria for PTSD. In the 12 months after beginning treatment, participants with PTSD did not engage in self‐harm more often or present with more severe borderline PD symptoms. However, a higher level of PTSD symptom severity at baseline was associated with more frequent self‐harm during follow‐up, incidence rate ratio = 1.03, 95% CI [1.01, 1.06], p = .005. Participants whose PTSD did not improve or worsened were more likely to still self‐harm, odds ratio (OR) = 0.91, 95 % CI [0.85, 0.97], p = .004, and report more severe borderline PD symptoms, b = 0.53, 95% CI [0.34, 0.73], p < .001, at the end of 12 months. The negative effects of PTSD symptoms remained significant after adjusting for potential confounders and were partially mediated by high ongoing levels of emotional dysregulation, indirect effect b = 0.32, 95% CI [0.19, 0.50]. These findings identify subgroups at risk of poorer outcomes from evidence‐based psychological treatments for PD and support the idea that treating PTSD may improve outcomes.