The merger will create the world's largest hotel company, uniting brands like Sheraton, Westin and St. Regis with the Ritz-Carlton and Courtyard. (Reuters)

Marriott International, the Bethesda-based hotel behemoth, said Monday it has agreed to buy Starwood Hotels & Resorts in a $12.2 billion deal that would create the world’s largest hotel company.

The combined hotelier, to be based in Bethesda, would own or franchise 5,500 properties and be the only company to have more than 1 million hotel rooms. The cash and stock purchase marks the largest hotel deal since the private equity firm Blackstone Group bought Hilton Worldwide for $26 billion in 2007.

“This is a transformative event for Marriott,” Arne M. Sorenson, Marriott’s president and chief executive, said Monday in a briefing with investors. “When we look at Starwood, we see many aspects of its business that complement Marriott.”

Specifically, Sorenson said Marriott was drawn to Starwood’s large international presence, strong rewards program and its popularity among younger travelers. The Stamford, Conn.-based company owns 11 brands, including St. Regis, W, Westin and Sheraton.

“To be successful in today’s lodging space, a wide distribution of brands and hotels across price points is critical,” Adam Aron, Starwood’s chief executive, said during the briefing with investors. “Today, size matters.”

The combined company, which would have 30 brands, 1.1 million rooms and nearly $20 billion in annual revenue, would cement Marriott’s lead as the world’s largest hotel company. Hilton Worldwide, based in McLean, would be the second-largest hotelier, with 4,400 properties, 731,000 rooms and $10.5 billion in annual revenue.

The U.S. hotel industry is dominated by a handful of large chains, and some analysts say the purchase of Starwood means there is one less choice for consumers.

“Losing Starwood with properties all over the spectrum is a significant loss in the competition,” said Jim Butler, head of global hospitality at the law firm Jeffer Mangels Butler & Mitchell in Los Angeles. “It is going to be a major change in the hotel industry.”

The sheer size of the deal may give pause to regulators, according to some antitrust experts.

“There is not a clear path that’s been charted by the Federal Trade Commission or by the courts on these deals in the past,” said Edward B. Schwartz, a partner at the law firm of Steptoe & Johnson. “I would imagine the FTC is going to want to take a close look at this.”

The last time two major hotel companies merged, he said, was in 2004 when MGM Mirage purchased Mandalay Resort Group for $7.65 billion. The FTC approved that deal but only after requesting a second round of information.

Marriott has purchased a number of hotel companies in recent years as it beefs up its presence outside the United States. In 2014, it acquired Cape Town-based Protea Hospitality Group for about $186 million, making it the largest hotel company in Africa. Earlier this year, it became the largest full-service hotelier in Canada with the $135 million purchase of Delta Hotels and Resorts.

“Marriott has been very aggressive in the past,” said David Loeb, senior analyst at Robert W. Baird & Co. “This is not out of character for them to build their brand by combining with another group.”

Loeb added a number of the company’s brands were the result of acquisitions, including Ritz-Carlton, Renaissance and Residence Inn.

The combination of the companies will make it easier, and more cost-effective, to operate existing hotels and open new ones, Sorenson said. Marriott expects to cut at least $200 million in annual costs by 2018 by merging operations.

“By being bigger, our hotels can be made to perform better,” Sorenson said.

Sorenson would remain president and chief executive of Marriott. The company’s board of directors would increase to 14 after adding three members of Starwood’s board.

Marriott has more than 2,000 employees at its Bethesda headquarters. Sorenson said it was too soon to tell how many employees may be added as a result of the deal.

“The only thing we are reasonably certain of is that we’ll keep our headquarters in the Washington, D.C., area,” he said in an interview. “There is no aspect of this deal that is a reconsideration of that.”

The company has plans to relocate its headquarters in the coming years, and Sorenson said he did not foresee any changes to the company’s search for a new campus.

The cash and stock deal is expected to be finalized in mid-2016, pending approval by shareholders and regulators. Starwood shareholders would receive 0.92 shares of Marriott’s stock, as well as $2 in cash, for each share of Starwood.

Both Marriott and Starwood have large loyalty rewards programs — with 54 million members and 21 million members, respectively — but the companies have yet to decide when, or how, those two programs will be combined.

An earlier version of this story incorrectly said the combined annual revenue of the two companies would be $2.7 billion. It would be nearly $20 billion. The file has been updated.