Developing and emerging country (DEC) currencies’ subordinate position in the international monetary system represents a fundamental constraint on sustainable development. The existing critical political economy literature on the asymmetric international monetary and financial system so far has largely focused on the macrostructural and financial constraints that both underpin and would need to change to increase monetary sovereignty in DEC. The role of payment infrastructures for international monetary power has received some attention in critical infrastructure studies. However, this literature largely focuses on the top of the monetary hierarchy, or interprets infrastructures as colonial devices, which impose systems developed in and for the Global North to subordinate financial spaces. This chapter brings those literatures together to ask whether and how local currency regional payments systems in DECs can be used to build monetary sovereignty from the bottom. Building on an extensive qualitative study of the local currency payment system SML between Argentina, Brazil, Uruguay, and Paraguay, it identifies the substantial operational, political, and macrostructural constraints on such initiatives which can – if at all – only be overcome with a comprehensive and multifaceted program to support the use of DEC currencies across all functions of money.