This article provides a critical interrogation of the Brazilian tax on foreign exchange derivatives deployed between 2011 and 2013. It analyses the drivers of the policy-making process that led to implementation of the measure, locates it within the broader policy response regarding the management of cross-border capital flows and speculative finance, and assesses its political economy significance in light of class dynamics. The author makes three arguments. First, this innovative policy tool must be interpreted in terms of the emergence of a specific form of state power allowing for the continuation of finance-led strategies of accumulation, while mitigating some of their worst consequences. Second, this form of state power internalizes the subordinate positionality of Brazil in the global financial and monetary system. Third, while financialization processes have eroded the efficiency of a number of policy tools, this policy experiment demonstrates the possibility of regulating complex financial markets, provided that appropriate resources are dedicated to the task, and that there is the political will to do so. The article concludes by discussing theoretical implications, for how to theorize state and financialization, as well as political implications.I would like to thank my interviewees for their openness and assistance. Thanks are also due to two anonymous reviewers for helpful comments on earlier drafts of this article. The usual disclaimers apply. Development and Change 50(5): 1310-1341.