The European Central Bank (ECB) emerged from the sovereign debt crisis as one of the most powerful supranational institutions. Against this background, this article explains how and why the ECB became empowered during the euro area crisis. Building on the delegation, governor’s dilemma, and epistemic community approaches, we argue that the ECB ability to play a strong role in this empowerment process and to convince member states to entrust it with more competences was the outcome of a combination of three factors: limited cohesiveness within the collective principal (Eurogroup); a fiduciary relationship characterized by broad discretion and independence on the trustee side (ECB); and strong specialization with the ECB acting as epistemic entrepreneur. We illustrate our argument with two cases: the Trichet letters exemplify an autonomous emergency empowerment and the introduction of the single supervisory mechanism demonstrates ECB influence on institutional design decisions in negotiating processes.