Card Security Code The card security code is a unique three or four digit number, separate from your credit card number. Your security code can be found in the following locations on the following cards: Visa/MasterCard/Discover Your card security code for your MasterCard, Visa or Discover card is a three-digit number on the back of your credit card, immediately following your main card number. American Express The card security code for your American Express card is a four-digit number located on the front of your credit card, to the right or left above your main credit card number.