An American Express card’s security code is 4 digits long.
American Express calls this four-digit code the card identification number, or CID. It is printed on the front of the card, usually above and to the right of the account number, rather than on the back signature panel used by many other networks.
The code is separate from the card number and expiration date. Merchants request it mainly for card-not-present transactions, such as online, telephone, or mail orders, because it provides an additional check that the buyer has access to the physical card. It is printed rather than embossed and is not encoded in the magnetic stripe.
The most common mistake is assuming every card uses a three-digit CVV or CVC. Visa, Mastercard, and many Discover cards normally use three digits on the back, while American Express traditionally uses four digits on the front. American Express may also use a three-digit code on the back for some purposes, but the familiar Amex security code asked for in checkout is the four-digit CID.