The $13.6bn (£9.5bn) deal will see the total number hotels owned by the company taken to 5,700, with more than 1.1m rooms across 30 brands in 110 countries.
The acquisition was delayed last month after the two companies announced that they had agreed to extend the time period for the Chinese Minister of Commerce (MOFCOM) to complete its review of the merger.
The company has now merged the Starwood Preferred Guests (SPG) and Marriott Rewards loyalty programmes, meaning guests can redeem their points across the entirety of the company’s expanded portfolio.
John Marriott jr, executive chairman of Marriott International, said: “With the addition of Starwood’s strong brands, great properties, and talented people, we have dramatically expanded our ability to provide the best experiences to our customers.
“We also welcome the tremendous responsibility as the world’s largest hotel company to be a good global steward, providing new opportunities for our associates and building the economic strength of the communities we call home.”
Arne Sorenson, president and CEO, added: “We can now provide a better range of choices for our guests, more opportunities for our associates, and greater financial benefits for our owners, franchisees, and shareholders.”