Trump to visit ill brother in New York: White House
Spokesman Judd Deere said that Trump, who is traveling Friday to his golf club in nearby Bedminster, New Jersey, will go into New York City for the visit.
US media reports say that the brother is seriously ill, although there were no details.
Trump told reporters only that "he's having a hard time."
While far less famous than his older brother, Robert Trump has long been an integral part of the family real estate empire and is fiercely loyal to the president.
Robert Trump unsuccessfully went to court to try and get an injunction preventing publication of a book by his niece Mary Trump, called "Too Much and Never Enough: How My Family Created the World's Most Dangerous Man."
The book, which paints Donald Trump as the product of a "toxic" family was a "disgrace," Robert Trump said. Although a temporary restraining order was applied, a judge lifted this in July, allowing publication.
At a press conference, President Trump called his brother "wonderful" and said he and his brother have "had a great relationship for a long time, from day one."
Trump is also due to make a speech to the New York police officers' union on Friday.