Donald Trump: Steve Bannon Has 'Lost His Mind' Suggesting Russia Meeting Was Treason

Donald Trump has completely turned on Steve Bannon -- saying the guy has lost his mind ... this after Bannon called a meeting the President's son took with Russians "treason."

Bannon was quoted by author Michael Wolff in a sweeping new book detailing the alleged chaos from Trump's first year in office as saying Don Jr.'s meeting in 2016 was treasonous.

Trump responded Wednesday in an official statement saying ... "Steve Bannon has nothing to do with me or my Presidency. When he was fired, he not only lost his job, he lost his mind."

The Prez goes on to say that Bannon had little to do with his historic victory, and that while Bannon claims to be at war with the media ... he often leaked from the White House.

Trump says Bannon helped people -- presumably Wolff -- write phony books with his insights, which Trump says were actually limited.