The fine is expected to be announced on Monday, the source said, declining to be identified because it is not yet public.
The development comes as governments around the world crack down on tax evasion as they look to close widening budget gaps fueled by economic weakness.
The centerpiece of the U.S. tax recovery effort has been its legal case against Swiss banking giant UBS AG
A Citigroup representative was not immediately available for comment.
FINRA declined to comment.
The Financial Times first reported the news.
(Reporting by Paritosh Bansal; Editing by Jan Paschal)