The euro is the official currency in Martinique. U.S. dollars are accepted in some hotels but generally at an unfavorable rate. You can usually get the best rate by withdrawing euros from ATMs. Banks no longer exchange currency or traveler's checks; however, bureaux de change such as Change Caraïbes—which has offices at the airport and in Fort-de-France (14 rue Victor Hugo and Le Bord de Mer)—still will exchange both and usually offers fair rates.
You can find ATMs at the airport and at branches of the Crédit Agricole bank, which is on the Cirrus and Plus systems and accepts Visa and MasterCard.
Major credit cards are accepted by most hotels and restaurants, but some establishments accept neither credit cards nor traveler's checks, particularly once you get away from Fort-de-France and Pointe du Bout. Many establishments don't accept American Express in any case.