Abstract Background Patients with borderline personality disorder (BPD) experience significant affect regulation difficulties that cause serious consequences in their work, emotional, and social environments. This dysfunctional pattern also produces great suffering and a heavy burden on their relatives. Fortunately, some studies show that treatment of relatives of people with BPD begins to be important in the patients’ recovery and in improving family dynamics. One of the treatments that has obtained the most empirical support is Family connections (FC). This 12-session program is an adaptation of different Dialectical Behavior Therapy strategies. To test the efficacy of FC, five uncontrolled clinical trials were conducted, with pre-post treatment and follow-up assessments. The results of these studies and subsequent replications showed an improvement in family attitudes and caregiver burnout. Our research team adapted FC for delivery in the Spanish population. We intend to test the efficacy of this program versus a treatment as usual condition. Moreover, we aim to test the efficacy of this program and study its effectiveness (in terms of participants’ acceptance). This paper presents the study protocol. Methods The study is a randomized controlled trial. The participants will be recruited in a Personality Disorders Unit and randomly assigned to one of two treatment conditions: Family Connections group (FC) or Treatment As Usual (TAU). Primary outcome measures will be the BAS and FAD-GFS. Secondary outcomes will include DASS-21, FES, GS, and QLI. Participants’ treatment acceptance and degree of satisfaction will also be measured. Participants will be assessed at pre-, post-treatment, and 6-month follow-up. Intention to treat and per protocol analyses will be performed. Discussion This is the first study on FC for relatives of people with borderline personality disorder (BPD) compared to an active condition (TAU), and this is the first time relatives’ and patients’ data will be analyzed. In addition, it is the first study to test the efficacy of the program in Spain. This intervention could contribute to improving the efficiency and effectiveness of current treatment programs for relatives of people with BPD, help to decrease burden, and improve the family connection. Trial registration ClinicalTrials.gov ID: NCT04160871. Registered November 15th 2019.