Trump's honesty on the golf course has been called into question many, many times in the past ... with former golf partners like Samuel L. Jackson and Oscar De La Hoya flat out saying they witnessed Trump cheat.

Rick Reilly even wrote a book -- a freakin' book -- about Trump's alleged cheating.

But, John Daly -- who's been friends with the Prez for years and golfed with him at Trump National in NJ on Monday -- tells TMZ Sports, it just ain't true.

"It's funny how these people say, 'does he cheat?' Well, when he goes out and plays, if he hits a bad shot, he might hit a Mulligan, but he doesn't count it when we're playing the match."

For those not down with the golf lingo ... a mulligan is essentially a redo after a bad shot.

The 2-time Majors winner doubled down, and called out someone else who was in the Oval Office.

"It's just amazing that people call him a cheat in golf. You wanna call a cheat in golf? I'll tell ya, Bill Clinton, he took a Mulligan on putts, chips, when I played with him."

"I don't think Bill Clinton could've broken 100. At least the President, Mr. Trump, he can shoot 80. 78 to 84, probably."

There's more ... Daly gives us his U.S. Presidents golfer power rankings -- and tells us what him and Trump discussed on the links.