Power is a central concept in relationships, yet existing self-report measures of relationship power are not well validated and do not assess all aspects of power. To address this, we developed the Relationship Power Inventory (RPI), a self-report measure of power for romantic partners. In Study 1, we identified the most important decision-making domains in romantic relationships. In Study 2, we generated an item pool assessing relationship power, selected the best performing items for inclusion, and tested the convergent and divergent validity properties of the RPI. Study 3 revealed RPI scores predict observer ratings of power during decision-making discussions and showed the RPI has good test-retest reliability. We discuss how the RPI can advance research on power in close relationships.