Background Couple relationship distress is common and associated with poor physical, psychological, and relational outcomes for both partners. Emotionally Focused Therapy for couples (EFT) is a short-term structured approach based on attachment theory that integrates a humanistic, experiential approach to restructuring emotional experience and a systemic structural approach to restructuring interactions. This model has been shown to be an effective treatment for couple distress. The supporting research, however, has only been conducted with English-speaking couples. Despite Spanish being the second-most spoken language and meaningful cultural differences between English- and Spanish-speaking countries, the efficacy of EFT has not been examined in this cultural context. This study will examine the efficacy of EFT in this particular context and advance the understanding of potential mechanisms of change. Methods We will use a multicenter randomized wait-list controlled design to examine the efficacy of EFT in a Spanish-speaking sample of moderately distressed couples. One hundred forty individuals in 70 couples in Argentina, Costa Rica, Guatemala, Mexico, and Spain will be randomly assigned to receive 19-21 sessions of EFT or be placed on a waitlist. Outcomes on a range of relational and individual mental health variables will be assessed prior to random assignment, throughout treatment, and at the conclusion of treatment. Primary outcomes will include dyadic adjustment, couple satisfaction, and attachment. Secondary variables, such as loneliness, parenting, affective communication, and sexual satisfaction, will be included as potential mediators of the treatment effect. Couples in the treatment group will also be assessed at 3-, 6-, 12-, 18-, and 24-month follow-ups. Process variables such as the therapeutic alliance will also be assessed routinely in couples assigned to the treatment group. Couples in the waitlist will receive a psycho-educational program based on EFT after completing the study. Discussion This study will be the first RCT of Emotionally Focused Therapy in a Spanish-speaking context. The results of the study will inform researchers interested in whether treatments developed and tested in the USA and Canada can be effective in differing cultural contexts. It may also point researchers and clinicians to areas where cultural adaptation is needed to improve efficacy.