"You know, my husband and I have been married 28 years and we have filed separate tax returns for 28 years," McCain told "Today." "This is a privacy issue. My husband is the candidate."
McCain, an heiress to a major beer distribution company who is thought to be worth around $100 million, said she had no plans to release her returns even if she becomes first lady.
During the 2004 campaign, Teresa Heinz Kerry, the wealthy wife of Democratic presidential nominee John Kerry, came under pressure to release her returns from newspaper editorial pages and conservative critics. She eventually released part of her returns.
The New York Times has called on Cindy McCain to release her returns, as it did with Teresa Heinz Kerry four years ago.
"There is no question that Mr. McCain benefits from his wife's money, including his low-cost use during the campaign of a corporate jet owned by a company headed by Mrs. McCain," the newspaper wrote.
It added: "Mrs. McCain, the daughter of a multimillionaire Anheuser-Busch distributor, is not the candidate, but the need to gain public trust and to air potential conflicts of interest is vital."
John McCain recently released two years worth of tax returns, and reported a 2007 income of roughly $405,000. He and his wife signed a prenuptial agreement before they were married. Barack Obama and Hillary Clinton file jointly with their spouses and have released their returns.