This paper highlights that banks shape firms’ currency choice in trade, and that a central bank can internationalize its currency by changing foreign banks’ incentives to build balance sheets in it. Using Korean exporter–partner-country–year invoicing data for 2006–2019, the authors find that the Korea–China central bank swap line raised RMB invoicing among Korean exporters, largely at the dollar’s expense, while the expiry of the Korea–Japan line was followed by a fall in yen invoicing. Four pieces of evidence link these shifts to banks: Korean banks raised RMB deposit rates and cut yen rates after the respective events; firms whose main bank offered a wider destination-currency premium over the dollar invoiced more in that currency; banks’ foreign-currency de-posits, liabilities, and assets moved with the swap line events; and exporters whose main banks had greater pre-existing branch exposure to China shifted furthest toward the RMB. A model in which swap lines improve banks’ management of currency risk rationalizes these patterns.