Robert Trump, the younger brother of the US president, has died aged 72, the White House has confirmed.
"He was not just my brother, he was my best friend," Donald Trump said in a statement on Saturday.
The president had visited his brother in hospital in New York on Friday afternoon, telling reporters: "He's having a hard time."
US media reports had said Robert Trump was seriously ill, but it is unclear what illness he was suffering from.
Robert Trump used to manage real-estate investments for the Trump property empire.
He recently went to court in a failed bid to stop publication of his niece Mary Trump's tell-all book about the president, How My Family Created the World's Most Dangerous Man.
According to the New York Post, Robert spent more than a week in the intensive-care unit of Manhattan's Mount Sinai Hospital in June.