The president announced the death of his brother late Saturday.
"It is with heavy heart I share that my wonderful brother, Robert, peacefully passed away tonight," he said in a statement. "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."
He was 71 years old, according to The Associated Press.
Donald Trump reflected on the relationship he had with his brother, who is only two years younger, at a briefing on Friday.
"I have a wonderful brother, we’ve had a great relationship for a long time, from day one," he said. "It’s a long time ago. And he's in the hospital right now. And hopefully he'll be all right, but he is pretty -- he’s having a hard time."
Robert 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.
Trump referred to bis brother in the past tense at the rally, saying, "I just want to pay my respects to a really good guy that I love. He respected you."
Robert Trump was hospitalized in June in the intensive care unit at Mount Sinai Hospital in New York for more than a week.
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 was married to Ann Marie Pallan in March, according to the New York Post.
Robert Trump filed for divorce from his first wife, Blaine, in 2007. The two were married for 23 years.
ABC News' Elizabeth Thomas contributed to this report.
Editor's Note: This story has been updated to reflect that Robert Trump was 71 years old. An earlier version of the story indicated he was 72.