The President of the United States described the donor merely as a patriot and a friend when he announced the anonymous donation on Thursday.
The country has, due to a blocked budget in Congress, been in shutdown for almost a month. Despite this, Trump has promised that the military, one of the many professional groups affected by missing paychecks, will still get paid.
Trump has ordered the Pentagon to use unused research grants to cover the salary costs. But the donated amount has also been earmarked for soldiers' salaries, even though it only covers about $100 per employee.
Two people have, according to the New York Times, identified the donor as businessman Timothy Mellon, heir to the Mellon bank fortune, who is also described as a "railway magnate".
He usually keeps a low profile and has written in his autobiography that he moved to the state of Wyoming for "lower taxes and to be surrounded by fewer people", the newspaper writes.
Mellon is reported to have donated hundreds of millions to Trump and his party in recent years.
When journalists asked Trump about Mellon on Friday, the President only replied that he was a "great American citizen" and that he does not want any attention.
He prefers that his name not be mentioned, which is quite unusual in the world I come from, said Trump.
The New York Times has not been able to reach Mellon for a comment.
