Power dynamics are fundamental when negotiating conflicts. However, no instrument for measuring power in romantic relationships has been adequately adapted to Spanish culture. The goal of this research was to adapt the Relationship Power Inventory (RPI; Farrell et al., 2015) to Spanish culture and language, filling this gap by providing a rigorous instrument for evaluating this construct. Study 1 was conducted to obtain evidence based on Spanish adaptation of RPI content. Once the Spanish adaptation of the RPI was built and we obtained validity evidence based on the test content, in Study 2, the scale was administered to two different samples of the adult population following a cross-validation approach. Specifically, in Sample 1 (N = 400), the training sample, a statistical analysis and an exploration of the dimensional structure and reliability of the measure were carried out. In Sample 2 (N = 755), the validation sample, the internal structure of the scale was confirmed, and evidence of external validity and generalization was obtained. The exploratory and confirmatory factor analysis showed a good fit for the four-factor structure. These dimensions were invariant to gender and had adequate validity based on their relationship with other variables (dependence on the partner, conflict-resolution strategies, and psychological well-being). In sum, the Spanish version of the RPI (SARPI) is a reliable instrument with sufficient valid evidence to provide accurate measurement of power differences in the context of romantic relationships.