It's the first time the Republican-appointed leader of the federal judiciary has offered even a hint of criticism of Trump, who has previously blasted federal judges who ruled against him.
Supreme Court justices, and the chief in particular, hardly ever issue statements on news events.
The Chief Justice is an aggressive defender of the judiciary and has frequently expressed concern about attacks on its impartiality, whether they come from the left or the right. He had made it clear last month that he felt the recent partisan battle over the confirmation of Justice Brett Kavanaugh had cast a shadow on the Supreme Court.
Liberals who follow the court and are often critical of Roberts applauded his latest statement.
"A remarkable rebuke of a President by a Chief Justice - offhand, I can't think of any historical analogy even close," Georgetown law professor Marty Lederman said in a tweet.
"But then again, every day Trump breaches norms never before breached. Major kudos to Chief Justice Roberts for doing the right thing."
-AP, Washington Post