Abstract This contribution serves to provide a fully-fledged analysis of the compatibility of the Spanish windfall tax on credit institutions with EU law (EU monetary and banking policy, EU Fundamental Freedoms, and state aid) and international law (investment law and double tax treaties). Our analysis reveals that the design of the Spanish tax captures windfall profits that are large and easy to detect due to inflation and high interest rates. Although the tax is compatible with EU law, it compromises the area of bilateral investment treaties and bilateral tax treaties.