Under the terms, Citi, the world's largest issues of consumer credit cards, would become the exclusive issuer of Costco co-brand credit cards. Visa would replace American Express as the credit card network for Costco in the U.S. and Puerto Rico beginning next month.
Costco and American Express were not able to renew their partnership, which was set to expire at the end of March 2016. The partnership dates back 16 years.
In an interview with CNBC's "Squawk Box" Monday morning Warren Buffet said that "Costco thought they could do better elsewhere," in choosing not to renew its contract with American Express.
"Clearly if you have a retailer with the size and scope of Costco and you have the exclusive card, you know that is a very important relationship," he said.
Berkshire Hathaway owns more than a 10 percent stake in American Express, about $12.5 billion in shares. He also told CNBC that he loves the company and thinks it will continue to do well despite losing the Costco deal.
The Visa credit card would provide rewards to Costco members and serve as the company's membership card. Costco will provide additional information in the coming weeks for how customers can transition to the new card.
Visa, Citi and American Express were all trading in positive territory in premarket trading Monday.
—The Associated Press contributed to this report.
—CORRECTION: This version corrected the link to Citi's stock ticker.