Citigroup has bought its main Belfast premises from Titanic Quarter.
The financial services firm has rented the Gateway building since 2009 and said the decision to buy demonstrates a long-term commitment to Belfast.
The price has not been disclosed, but the building had been on the market for £34m.
Citi is one of Belfast's major employers with about 2,600 staff in the city.
About half those workers are accommodated in the Gateway which consists of three linked six storey blocks.
In 2017, the US firm gave a reassurance about the future of the its presence in Belfast after Brexit.
Citi's head of UK operations said: "There is no fundamental reason why Belfast's attractions to us should change as a result of Brexit.
"I don't see that standing in the way of either what we've got in Belfast or our plans to continue developing."