Wells Fargo (WFC) is joining the American Express (AXP) credit card network.

The San Francisco bank will issue new credit cards on the American Express network beginning in mid-2014, the two companies announced Wednesday. Financial terms of the agreement were not disclosed.

Wells will be a dual-network issuer, offering customers access to cards on the Visa (V) network, its current partner, along with American Express, a bank spokeswoman said. Wells will begin a pilot program for customers in select U.S. markets beginning in the third quarter of 2013 before the nationwide launch in the middle of next year.

As part of the agreement, American Express will develop a custom product line for Wells Fargo's customers, and AmEx's LoyaltyEdge will help Wells Fargo develop a loyalty program for the Wells cards.

"We want to give the customers who already carry our card - and the ones who don't yet - greater rewards and benefits for having a Wells Fargo card in their wallets," said Avid Modjtabai, head of Wells Fargo's consumer lending group, in a news release. "The breadth of our offerings, our strong retail network and the power of this new partnership with American Express position us well to continue to deliver compelling choices and experiences to our customers."

Wells Fargo is the eighth-largest credit card issuer by purchase volume, with $56.2 billion in sales in 2012, according to CardHub, a personal-finance website. Approximately one-third of Wells' 70 million customers have credit cards, Beverly Anderson, Wells Fargo's executive vice president for consumer credit solutions, told CNBC on Wednesday.