Contextual therapy posits that a truly comprehensive grasp of human existence is composed of individual and relational realities. The Relational Ethics Scale (RES) is the only validated instrument to measure relational ethics, one of the relational realities' four dimensions, which was theorized by Boszormenyi-Nagy. The RES measures people's perception about constructs of loyalty, entitlement, trust and justice in their family of origin and in their current and significant relationships. Relational ethics has been shown to have implications for mature development, psychological health and family functioning. The aim of this research was to adapt the RES for use with Spaniards residing in Spain (N = 1181). Psychometric properties were analyzed, and construct validity was assessed using the Spanish-Differentiation Self Inventory. Results obtained indicate the Spanish Relational Ethics Scale (S-RES) is a valid and reliable tool for relational ethics assessment in Spanish populations. Couple and family counselors in Spain may consider the clinical assessment of relational ethics using the S-RES in order to explore issues of fairness, trust, roles, and re-balance when working to support complex family systems. Future research directions using the newly validated S-RES are discussed.