Citigroup bought its major Belfast facilities from the Titanic Quarter.
The financial services company has rented the Gateway building since 2009 and said the decision to buy shows a long-term commitment to Belfast.
The price was not disclosed, but the building was on the market for £ 34 million.
Citi is one of Belfast's leading employers, with about 2,600 employees in the city.
About half of these workers are accommodated in the Gateway, which consists of three interconnected six-story blocks.
In 2017, the US company gave a guarantee on the future of its presence in Belfast after Brexit.
Citi's chief operating officer in the UK said: "There is no fundamental reason why Belfast's attractions for us should change as a result of Brexit.
"I do not see it in the way we have in Belfast or our plans to continue developing."