In economics, an optimum currency area (OCA) or optimal currency region (OCR) is a geographical region in which it would maximize economic efficiency to have the entire region share a single currency.

The underlying theory describes the optimal characteristics for the merger of currencies or the creation of a new currency. The theory is used often to argue whether or not a certain region is ready to become a currency union, one of the final stages in economic integration.

An optimal currency area is often larger than a country. For instance, part of the rationale behind the creation of the euro is that the individual countries of Europe do not each form an optimal currency area, but that Europe as a whole does. The creation of the euro is often cited because it provides the most modern and largest-scale case study of an attempt to identify an optimum currency area, and provides a comparative before-and-after model by which to test the principles of the theory.

In theory, an optimal currency area could also be smaller than a country. Some economists have argued that the United States, for example, has some regions that do not fit into an optimal currency area with the rest of the country.

The theory of the optimal currency area was pioneered in the 1960s by economist Robert Mundell. Credit often goes to Mundell as the originator of the idea, but others point to earlier work done in the area by Abba Lerner. Kenen (1969) and McKinnon (1963) were further developers of this idea.