Correction: An earlier version of this article misstated the year in which Capital One completed its acquisition of Chevy Chase Bank. It was 2009, not 1999. This version has been corrected.

Capital One Bank announced a $9 billion deal Thursday to acquire the online bank ING Direct USA, accelerating the McLean firm’s transformation from a credit card lender to a mainstream consumer bank.

The acquisition catapults Capital One from being the eighth-largest U.S. bank measured by deposits to the fifth — bigger than U.S. Bancorp and just below Citigroup. It also makes Capital One the country’s largest online bank, putting it at the forefront of an industry evolution that targets younger customers.

The deal allows Capital One to expand at a time when consumers are reluctant to take on new debt, said Matt McCormick, an analyst at Bahl & Gaynor Investment Counsel. Banks traditionally grow by making more loans, McCormick said. “The other way to grow is through acquisitions.”

The acquisition rounds out Capital One’s portfolio, potentially allowing it to lend more and enter new markets, McCormick said. “It adds more stability for Capital One, which is known for their credit card prowess,” he said. “It separates them from a lot of their peers. It gives them size.”

This is the latest aggressive move by Capital One to expand beyond the credit card business. In recent years, it has made acquisitions that gave it hundreds of branches in the South and Northeast, including Louisiana and New York. In 2009, Capital One completed its acquisition of Chevy Chase Bank, a deal that gave it a sizeable presence in the Washington region, with nearly 250 branches.

This acquisition is one of Capital One’s largest. With the deal, the company’s online banking business will more than triple, to $109 billion, with the addition of ING’s $80 billion in deposits. Capital One is also spotlighting the desirability of ING’s 7 million customers, many of whom are younger and more wealthy than its own. Sixty-nine percent of ING’s customers are 47 or younger, 10 percent more than Capital One’s.

“The acquisition of ING Direct is a game-changing transaction,” Richard D. Fairbank, Capital One’s chairman and chief executive, said in a statement. “Its innovative platform and customer focus are well aligned with Capital One’s own vision.”

Capital One, which sold its home-loan origination unit in 2007, is also reentering the mortgage market with this acquisition. ING Direct has about $41 billion in mortgage loans.

Capital One is buying the company from the Dutch financial services firm ING Groep NV for $6.2 billion in cash and $2.8 billion worth of stock, which will give ING a 10 percent stake in Capital One. An ING representative will also be added to Capital One’s board. The deal could close by the end of the year or in early 2012, according to statements from both firms. It is subject to approval by government regulators in the United States and the Netherlands.

For ING Groep, the deal satisfies a European Commission directive that it sell its U.S. business in return for government aid during the financial crisis.

“Although I regret that ING Direct USA will no longer be a part of ING, I am very pleased that we have found in Capital One a good home for our customers and employees,” Jan Hommen, ING Groep’s chief executive, said in a statement. “In addition, the transaction today shows ING is taking decisive steps in the restructuring of ING Group and underlines our commitment to meet the requirements of the EC in a prudent yet decisive manner.”