Mr Bolton sat for an interview with ABC News that aired on Sunday to promote his new book, The Room Where It Happened, which hits shelves on Tuesday.
Asked by ABC's Martha Raddatz whether Mr Trump "really believes Kim Jong-un loves him," Mr Bolton responded that he doesn't know of any other explanation.
"I think Kim Jong-un gets a huge laugh out of this. I mean, these letters that the president has shown to the press — off the record and whatnot, but I've been in the room when he's done it — are written by some functionary in the North Korean Workers Party Agitprop Office," Mr Bolton said, referring to personal letters the North Korean leader has supposedly written to Mr Trump.
"And yet, the president has looked at them as evidence of this deep friendship. Even if it were a deep personal relationship, it doesn't change the fact Kim Jong-un is never going to give up his nuclear weapons program. And from the U.S. national security point of view, that is the only thing that matters," Mr Bolton said.