Read more: Donald Trump's golf game tells us an awful lot about him
The president has claimed he has achieved scores of 68, which would class him as a highly skilled golfer, particularly at the age of 71.
But Pettersen said although "golf is the only thing the man thinks about", he is not quite as skilled as he would like people to believe.
"He cheats like hell, so I don't quite know how he is in business," she told Norwegian newspaper Verdens Gang.
Does Donald Trump Cheat at Golf?"I'm not going to say Donald J. Trump cheats at golf, but..."
Accused of cheating before, Trump has said he has too much intergrity to do such a thing: http://bit.ly/2hPTkWJ
Posted by Golf Digest on Friday, December 16, 2016
"They say that if you cheat at golf, you cheat at business."
Revealing some of his techniques, she said Trump often opts to add a shot without actually taking his final putt, probably because his putting is not very good.
She said: "He always says he is the world's best putter. But in all the times I've played him, he's never come close to breaking 80.
"But what's strange is that every time I talk to him he says he just golfed a 69, or that he set a new course record or won a club championship some place. I just laugh."
Pettersen said that she got on well with Trump because she does not take "everything he says literally".
"You'll never meet another person who loves himself as much as he does," she said, "but I know how much he cares."
Before he was president, she added: "He called me once a month and asked how things were going. It was always about golf."
Trump has also been extremely helpful when she asked him for business advice, she said.
The 36-year-old said whenever Trump appeared he would discuss the golf matches they had played against each other.
She added: "And then he would say that [Norwegian businesswoman] Celina [Midelfart] was his greatest love. He has told that same story a hundred times. If you would ask her, the story is no doubt completely different."
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