The President released the following statement:
"It is with heavy heart I share that my wonderful brother, Robert, peacefully passed away tonight. He was not just my brother, he was my best friend. He will be greatly missed, but we will meet again. His memory will live on in my heart forever. Robert, I love you. Rest in peace."
Robert, 71, worked as a real estate developer and executive for the Trump Organization.
Donald Trump visited his ailing brother at New York Presbyterian Weill Cornell Medical Center in New York City on Friday. He made the detour to visit Robert prior to speaking to New York City Police Benevolent Association at his Bedminster, New Jersey, golf club.
The president did not say how long his brother had been hospitalized or why.
"I have a wonderful brother," President Trump said at a press briefing Friday. "We've had a great relationship for a long time, from day one. It's a long time ago. And he's in the hospital right now. And hopefully he'll be alright, but he is pretty -- he's having a hard time."
White House officials said on Friday that Robert Trump was seriously ill. He had spent a week in the intensive care unit at nearby Mount Sinai Hospital in June.
Democratic presidential candidate Joe Biden also tweeted about Robert Trump's passing.
Mr. President, Jill and I are sad to learn of your younger brother Robert’s passing. I know the tremendous pain of losing a loved one — and I know how important family is in moments like these. I hope you know that our prayers are with you all.— Joe Biden (@JoeBiden) August 16, 2020
Robert Trump was the youngest of five children by Fred and Mary Anne Trump, along with the president, Fred Trump Jr., Maryanne Trump Barry and Elizabeth Trump Grau. Fred Trump Jr. died in 1981.
He recently filed a lawsuit on behalf of the Trump family seeking to stop publication of a tell-all book by the president's niece Mary titled "Too Much and Never Enough."
Once a regular bold face name in Manhattan's social pages, he has kept a lower profile in recent years. He married his longtime girlfriend, Ann Marie Pallan, in March.
He divorced his first wife, Blaine Trump, more than a decade ago.
(The Associated Press contributed to this report)