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This article is about the medieval and early modern French currency, not the European Currency Unit (ECU) or an electronic control unit (ECU).

The first écu, issued by Louis IX of France, in 1266.

The term écu may refer to one of several French coins. The first écu was a gold coin (the écu d'or) minted during the reign of Louis IX of France, in 1266. Ecu (from Latin scutum) means shield, and the coin was so called because its design included a shield bearing a coat of arms. The word is related to scudo and escudo. The value of the écu varied considerably over time, and silver coins (known as écu d'argent) were also introduced.

Louis XIII AR Douzième d'Écu (21mm, 2.26 g). Paris mint, dated 1643.

In the 17th and 18th centuries, the name écu was applied exclusively to a large silver coin (introduced by Louis XIII in 1640) initially worth three livres tournois. From 1690 to 1725 rates were unstable and new écus were issued, and existing écus revalued. After 1726 the final écu remained stable at six livres tournois. The silver écu (sometimes also called the louis d'argent) was further broken down into a 1/4 value coin (the quart d'écu) and a 1/2 value coin (the demi-écu). For more on the 17-18th century currency system, see Louis (coin), livre tournois and Italian scudo.

The écu disappeared during the French Revolution, but the 5 francs silver coins minted throughout the 19th century were but the continuation of the old écus, and were often still called écu by French people.

The écu, as it existed immediately before the French Revolution, is approximately equivalent (in terms of purchasing power) to 20 euros or 25 dollars in 2006.

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The fact that the coin's name was the same as ECU, the abbreviation of the European Currency Unit, may have assisted the ECU's adoption by France despite its being an acronym of English words.