Robert had fallen seriously ill in a New York City hospital and was visited by his brother on Friday.
Mr Trump said: “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.”
The youngest of the Trump siblings had remained close to the 74-year-old president and, as recently as June, filed a lawsuit on behalf of the Trump family that unsuccessfully sought to stop publication of a tell-all book by the president’s niece, Mary.
Both longtime businessmen, Robert and Donald had strikingly different personalities.
Robert Trump began his career on Wall Street working in corporate finance but later joined the family business, managing real estate holdings as a top executive in the Trump Organisation.
Family biographer Gwenda Blair said: “When he worked in the Trump Organisation, he was known as the nice Trump.
“Robert was the one people would try to get to intervene if there was a problem.”
Robert Stewart Trump was born in 1948, the youngest of New York City real estate developer Fred Trump’s five children.
The president, more than two years older than Robert, admittedly bullied his brother in their younger years, even as he praised his loyalty and laid-back demeanour.
“I think it must be hard to have me for a brother but he’s never said anything about it and we’re very close,” Donald Trump wrote in his 1987 bestseller The Art of the Deal.
“Robert gets along with almost everyone,” he added, “which is great for me since I sometimes have to be the bad guy.”
A Boston University graduate, Robert Trump later managed the Brooklyn portion of father Fred Trump’s real estate empire, which was eventually sold.
Once a regular boldface name in Manhattan’s social pages, Robert Trump had kept a lower profile in recent years.
He avoided the limelight during his elder brother’s presidency, having retired to the Hudson Valley.
But he described himself as a big supporter of the White House run in a 2016 interview with the New York Post.
“I support Donald one thousand percent,” Robert Trump said.
In early March of 2020, he married his longtime girlfriend, Ann Marie Pallan.
The president’s surviving siblings include Elizabeth Trump Grau and Maryanne Trump Barry, a retired federal appeals judge.
Authors Michael Kranish and Marc Fisher described Robert Trump as soft spoken but cerebral in Trump Revealed: The Definitive Biography of the 45th President.
They wrote: “He lacked Donald’s charismatic showmanship, and he was happy to leave the bravado to his brother, but he could show flashes of Trump temper.”
