The president visited his brother at a New York City hospital on Friday after White House officials said he had become seriously ill. Officials did not immediately release a cause of death.
"It is with heavy heart I share that my wonderful brother, Robert, peacefully passed away tonight," Donald Trump 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."
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.
Robert Trump had reportedly been hospitalized in the intensive care unit for several days that same month.
Both longtime businessmen, Robert and Donald had strikingly different personalities. Donald Trump once described his younger brother as "much quieter and easygoing than I am," and "the only guy in my life whom I ever call 'honey.'"
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 Organization.