Her account came to light after data collated by the International Consortium of Investigative Journalists (ICIJ) was published online for the first time.
More than 200,000 offshore account details from Mossack Fonseca, a Panamanian law firm, are now available.
"UK companies are required to publicly publish details of their shareholders and therefore do not give her the necessary anonymity required to protect her personal safety, which has been jeopardised in the past owing to such information being publicly available," Watson's spokesman said.
"Offshore companies do not publish these shareholder details."
Other famous names listed in the papers include the Duchess of York, The X Factor's Simon Cowell and Paul Burrell, the former butler to the late Diana, Princess of Wales.
Watson, who is worth £35 million, according to the Sunday Times, appears in two films this year, alongside Tom Hanks in The Circle, and in a new Disney version of Beauty and the Beast.
But the actress, who played Hermione Granger in the Potter films, said in February that she wanted to spend a year reading literature and developing HeForShe, a UN initiative to boost gender equality.
Yesterday she was among signatures on an open letter on the Telegraph website urging Sadiq Khan, the new London Mayor, to erect a statue in memory of the suffragettes in Parliament Square.
In other Panama Papers revelations, it emerged that BP signed a multimillion-dollar contract for work in Iraq in 2014 with Unaoil, a firm that had been linked to corruption allegations there.
BP confirmed it had hired Unaoil for engineering services.
Other figures who have previously come under scrutiny include British Prime Minister David Cameron, President Vladimir Putin of Russia, and Lionel Messi the footballer. Iceland's Prime Minister Sigmundur Gunnlaugsson resigned after being named in the papers.
