You can’t buy this kind of publicity.

On the eve of the opening of his critical-dud “Transformers” flick, kiss-and-tell hunk Shia LaBeouf admitted to having a fling with former co-star Megan Fox — while she was struggling to keep it together with then-boyfriend and future hubby Brian Austin Green.

LaBeouf was bluntly asked, in an interview published yesterday by Details magazine, if he ever hooked up with the world-famous sex symbol while filming their slam-bang thrillers.

He danced around the question before saying yes.

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“Look, you’re on the set for six months, with someone who’s rooting to be attracted to you, and you’re rooting to be attracted to them,” he told the mag.

LaBeouf’s chat didn’t make it clear if they got together during “Transformers” in 2007, or during the making of “Transformers: Revenge of the Fallen,” which debuted in 2009 — or both.

Presumably, the off-screen romps happened during Fox’s on-off courtship with “Beverly Hills, 90210” icon Green.

Fox and Green began dating in 2004 and survived several reported breakups before tying the knot a year ago. When asked about Green, LaBeouf could barely speak a coherent sentence.

“I don’t know, man. I don’t know. I don’t know. I don’t know,” LaBeouf said.

Details reported that LaBeouf stammered “I don’t know” exactly 12 times. His final explanation for the alleged Fox tryst: “It was what it was.”

He said it’s no accident he and Fox brought heat to their on-screen roles.

“I never understood the separation of work and life in that situation,” he said. “But the time I spent with Megan was our own thing, and I think you can see the chemistry on screen.”

Reps for LaBeouf and Fox, both 25, yesterday refused to discuss the report.

The romance, like Fox’s role in the big-money franchise, was doomed.

Fox grew to hate her role as sexy, car-loving tomboy Mikaela Banes, according to LaBeouf. She had a blowup with director Michael Bay last year and quit, so she doesn’t appear in “Transformers 3: Dark Side of the Moon.”

“She felt like a prostitute,” LaBeouf said of Fox and the actress’ view of her eye-candy role.

Fox isn’t LaBeouf’s only co-star romance.

He dated Carey Mulligan, his “Wall Street: Money Never Sleeps” co-star, for more than a year before ending it in October — apparently on good terms.

“I still love her,” he said. “I think she’s a f – -king awesome person and an incredible actress. We’re still pals. I wouldn’t take any of it back, and I don’t think she would either. It just ran its course.”

LaBeouf went off-stage to pick up his current gal pal, whom he scored with old-fashion honesty.

“I just saw her, thought she was attractive, and told her so,” LaBeouf said of Karolyn Pho, a 22-year-old stylist. “Just straight. I put it on the table, and either it works or it doesn’t.”