Amecican Express had accused credit card firms of "cartel" behaviour
American Express says it is to accept a payment of about $1.8bn (£0.9bn) from Mastercard after it sued the credit card giant and some large US banks.
In 2004 American Express filed a suit saying Mastercard, Visa and their member banks had illegally blocked it from the US bank-issued card business.
American Express had accused Mastercard of conspiring to prevent some banks from issuing its credit cards.
Mastercard agreed to make 12 quarterly payments of $150m to American Express.
Last year, the company reached a settlement with Visa in a similar suit.
"We are pleased to have reached a settlement that will enable us to keep our strong balance sheet intact," said Mastercard boss Robert Selander.
He said the deal removed "the uncertainty, time commitment and expense of a prolonged court case".
Mastercard had remained the sole defendant in the lawsuit.
American Express had claimed that an association of credit cards had carried out "anti-competitive practices" that had prevented 20,000 US banks from using Amex credit card products.
It had also accused the card associations of functioning "as a cartel".