Background: Difficulty in emotion regulation is a hallmark feature of patients with borderline personality disorder (BPD). Therefore, change in the frequency of certain patient's coping strategies-aiming at emotion regulation-are among the most promising mechanisms of change in treatments for BPD. In parallel, it was highlighted that therapist responsiveness significantly contributed to outcome across treatment approaches (Stiles, 2009). Based on a randomized controlled trial (Kramer et al., 2014), the present process-outcome mediation analysis aims at examining the patient's early change in frequency of coping strategies-in particular the decrease in behavioral forms of coping-as potential mechanism of change in responsive treatments for BPD. Method: A total of 57 patients with BPD were included in the present analysis, out of whom 27 were randomly assigned to a 10-session psychiatric treatment and 30 to a 10-session psychiatric treatment augmented with the responsive intervention of the motive-oriented therapeutic relationship (Caspar, 2007). The 1st, 5th, and 9th session of each therapy were transcribed and analyzed using the Coping Action Pattern Rating Scale (Perry et al., 2005; 171 sessions analyzed in total), a validated observer-rated method for assessing coping strategies in the therapy process. Psychological distress was assessed using the OQ-45 at intake, after Session 5, and after Session 10. Results: The results confirmed a responsiveness effect associated with the motive-oriented therapeutic relationship and showed a significant decrease in frequency of behavioral forms of coping, F(1, 54) = 3.09, p = .05, d = .56, which was not different between the 2 conditions. In addition, we demonstrated that the early decrease in behavioral forms of coping between Sessions 1 and 5 partially mediated the link between the group assignment and the change in psychological distress between Sessions 5 and 10. Conclusions: These results shed light on the centrality of therapist responsiveness in treatments for BPD and its impact on very early change in patient's in-session behavioral coping strategies, contributing to the effectiveness of short-term treatments for BPD.