Background: The literature reveals that borderline personality disorder (BPD) is an important public mental health problem that affects both the patients and their families. Moreover, studies indicate a high prevalence of psychological symptoms and burden in relatives of people with BPD. Therefore, it is necessary to develop useful and accessible interventions specifically addressed to the caregivers. Smartphone interventions with Ecological Momentary Assessment (EMA) and Ecological Momentary Interventions (EMI) offer several potential advantages in this regard. The aims of our study are to test the effectiveness of a combined intervention supported by a smartphone app versus the same intervention supported by a paper-based manual, studying the feasibility and acceptance of both conditions and evaluating the perceptions and opinions of families about both interventions. This paper contains the study protocol. Method: The design of this study protocol is a randomized controlled trial. A minimum of 116 relatives will be randomly assigned to two conditions: Treatment as usual (TAU) (N = 58) or Treatment as usual + EMI (TAU+ EMI) (N = 58), with TAU being the Family Connection program. The primary outcome will be the Burden Assessment Scale. Secondary outcomes will include psychological symptoms, mastery and empowerment, and resilience. Outcomes will be assessed from pre-treatment to post-treatment (3 months). Statistical analyses will be performed using Student's t-tests, mixed models (ANCOVA) and intention-to-treat analysis. Discussion: The results of this study will provide a basis for future EMA- and EMI-based application interventions for family members of people with BPD and family members of people with other mental disorders who could benefit from the skills taught.