"100 per cent of the people around" Trump — "senior advisers, family members, every single one of them, questions his intelligence and fitness for office," he said, adding that aides think he is a "moron" and an "idiot".
"Let's remember, this man who does not read, does not listen. So he's like a pinball, just shooting off the sides."
He said efforts to stop the book have helped him sell more copies and asked: "Where do I send a box of chocolates?"
White House Press Secretary Sarah Huckabee Sanders hit back in a separate interview on Fox and Friends where she said Wolff never interviewed the President despite "repeatedly begging" to see him.
She called the book "complete fantasy" and Wolff "a guy who made up a lot of stories to try and sell books."
Trump has also spoken out online, tweeting: "I authorised Zero access to White House (actually turned him down many times) for author of phony book!
"I never spoke to him for book. Full of lies, misrepresentations and sources that don't exist. Look at this guy's past and watch what happens to him and Sloppy Steve!"
However Wolff said on NBC he "absolutely" spoke to President Trump.
"Whether he realised it was an interview or not, I don't know, but it certainly was not off the record."
"I spoke to him after the inauguration, yes. And I had spoken to, I mean I spent about three hours with the president over the course of the campaign and in the White House so my window into Donald Trump is pretty significant," he said.
"I work like every journalist works so I have recordings, I have notes, I am certainly and absolutely in every way comfortable with everything I've reported in this book."
The publication of the book has rocked Washington with its revelations about Trump's family, marriage, germ phobias and behaviour in the Oval Office.
It has also marked a public break between Trump and his former chief of staff and campaign architect Steve Bannon, after Bannon called the Trump Tower meeting with Russians "treasonous".
It's also focused attention on Wolff, who has been described as a provocateur who loves the spotlight — except when it moves on.
"You think, well, what am I, chopped liver?" he said 2009 about coverage of his divorce, according to Women's Wear Daily.
The 64-year-old built his four-decade career writing about some of the world's rich and powerful people in seven books and across a wide range of newspapers and magazines. Sometimes, he critiqued the media. And often, he got scathing reviews back on his writing style, his focus on atmospherics and his factual mistakes.
Wolff acknowledges in the introduction of Fire and Fury that he could not resolve discrepancies in a White House riven by rivalries, however he told NBC he was "comfortable" with everything written in the book
"Many, in Trumpian fashion, are baldly untrue," Wolff writes of some accounts in the introduction.
"Those conflicts and that looseness with the truth, if not reality itself, are an elemental thread of the book."
He said he "settled on a version of events I believe to be true".
For example, Wolff writes in the book that Trump didn't know who former house speaker John Boehner was on election night, 2016.
Sanders disputes that, pointing to public photos that show the golf enthusiasts had hit the links over the years. Two people close to Boehner confirmed that and said they had spoken before and after the election.
The Washington Post also noted Wolff had been accused a number of times of "not just re-creating scenes in his books and columns, but of creating them wholesale."
Nearly a year ago, Wolff disparaged news outlets covering their own industry even in the time of Trump.
"The media should not be the story," he said on CNN in February.
Around the same time, Wolff also wrote a prescient Newsweek column about how the still-new and struggling Trump White House and the media might reach a balance or détente.
— With wires
