Stormy Daniels pours cold water on Donald Trump’s bedroom prowess in her new tell-all — and compares his privates to a popular Nintendo video game character.

The porn star, whose real name is Stephanie Clifford, goes well below the belt in “Full Disclosure,” a pre-sale copy of which was obtained by The Guardian.

Writing about her alleged tryst with the future president, Daniels says it “may have been the least impressive sex” she’d ever had.

In a blow to his manhood, she describes his genitals in lurid detail, saying his penis is “smaller than average,” but “not freakishly small.”

“He knows he has an unusual penis,” Daniels claims. “It has a huge mushroom head. Like a toadstool…

“I lay there, annoyed that I was getting f–ked by a guy with Yeti pubes and a d–k like the mushroom character in Mario Kart,” she adds in a description of his junk that lit up social media, where users assumed she was referring to Princess Toadstool’s trusty servant Toad.

“I mean, having your ding-a-ling compared to any Mario Kart character is far from ideal, but I think we can agree that Toad is without a doubt the worst,” @DanaWessel tweeted.

“Stormy Daniels compares Trump’s penis to Toad from Mario Kart and can someone please stop this planet I want to get off,” UK gaming writer Mark Brown added.

Daniels also disses Trump for his performance in the sack.

“It may have been the least impressive sex I’d ever had, but clearly, he didn’t share that opinion,” she writes.

The vixen, who alleged having a one-night stand with The Donald in 2006, adds that whenever she saw him on TV for years afterward, she’d say to herself: “I had sex with that … Eech.”

Even Trump’s ex-wife Marla Maples has refused to confirm that he was indeed the “best sex I’ve ever had” — as captured in a classic front-page headline in The Post in 1990.

“That was meant to be private,” Maples told the tabloid in February when she appeared for jury duty in Manhattan.

“I never said that, someone else said that. [But] is it true? I’m not going to talk about that. The truth will come out, just not here,” she said.

In her book, which goes on sale Oct. 2, Daniels also writes that Trump offered her a spot on his reality TV show “The Apprentice” — and suggested she could cheat in order to advance on the competitive reality TV show.

“We’ll figure out a way to get you the challenges beforehand,” Daniels writes.

“‘And we can devise your technique,’” Trump told her, she claims. “He was going to have me cheat, and it was 100 percent his idea.”

Daniels also expresses her astonishment that Trump scored primary wins in 2016 for the GOP nomination.

“It will never happen, I would say,” she writes about her response to former castmates who shared their disbelief that Trump might win the White House. “He doesn’t even want to be president.”

Daniels has been cooperating with federal prosecutors in Manhattan probing the $130,000 hush money payment she received on the eve of the election to keep mum about their alleged tryst.

The president has denied any affair with Daniels, although he admitted to personally reimbursing his personal lawyer, Michael Cohen, for her payoff.

Last month, Cohen pleaded guilty to campaign finance violations and other charges. He now wants to tear up the non-disclosure agreement he made with Daniels and get his $130,000 back.

Daniels’ forthcoming book arrives on the heels of “Fear,” famed Watergate journalist Bob Woodward’s damning book in which current and former White House aides paint a scathing picture of Trump as a thoroughly inept and unfit commander-in-chief.

Trump has slammed the book as a work of fiction.

Daniels also described the first time she laid eyes on a Trump cap during a celebrity golf tournament in Lake Tahoe, California, in 2006.

“He had a red cap, a Trump crest as a placeholder for the MAGA slogan not one of us could see coming,” she writes.