Katie Kelly had been trying for months to take her fiancé to a musical. But Matt Welton — a 34-year-old action-movie buff and Yankees fan from the Bronx — wasn’t interested.

Then a certain boxer’s name went up in lights, and suddenly, Welton was the one dragging her to the theater.

“All right, ‘Rocky’?” he said to Kelly, who’d never even seen Sylvester Stallone’s 1976 Oscar winner — or any of its five sequels — when a co-worker offered her tickets. “I’m into it.”

Welton’s not alone. “Rocky,” the musical, doesn’t open till Thursday, but it’s already drawing an audience Broadway rarely sees — guys more likely to be grabbing hot wings at a sports bar than perusing a Playbill.

“You get the look and feel that you’re actually at a boxing match,” Welton enthused afterward. “I was kind of shocked by that.”

Sure, there’s music — but who can resist singing along to “Eye of the Tiger”?

Welton and his compatriots haven’t gone unnoticed. “Audience is full of BIG men,” tweeted Jennifer Mudge, a “Rocky” cast member. “Cheering, sometimes weeping.”

Ted Boynton, 53, of Milford, Conn., rushed into the Winter Garden Theatre Wednesday night, his hands jammed in the pockets of his windbreaker.

Still, the high-school football coach stopped and smiled when asked about “Rocky,” the film.

“I probably saw it like four or five times,” he says. His wife, Cindy, gleefully jumps in.

“Ted has a ‘Rocky’ action doll at home, he has a T-shirt,” she says. “I’m surprised he’s not wearing the shirt!”

Boynton says he’d been looking forward to the show — his first since “Wicked” last year — since his wife bought him tickets for Christmas.

“I knew what the end result was going to be and still I was on the edge of my seat, just like the first time I ever saw [the movie],” he said afterward. “Wicked” didn’t hold his attention nearly as much.

The show’s crowning achievement — the one that had men (and a few women) leaping to their feet at a recent preview — is the Act 2 showdown between Rocky (Andy Karl) and Apollo Creed (Terence Archie). The first few rows of the orchestra empties out to make way for a boxing ring, and a Jumbotron descends from the ceiling.

Dudes reach out to slap Rocky’s hand as he charges down the aisle in his fight robe. By the time Adrian (Margo Seibert) runs through the crowd to embrace her man, the Winter Garden feels less like a Broadway house and more like the Philadelphia Spectrum.

The show is winning over celebrity fans too: Talking to Jimmy Fallon on “The Tonight Show,” Paul Rudd rhapsodized about taking his 9-year-old son and his friends to the first night of previews, when Stallone himself popped up in the ring. (Don’t expect that to happen all the time.)

“They were mesmerized,” Rudd said. “It was so great.”

Steven Van Zandt — Little Steven of Bruce Springsteen’s E Street Band and Silvio Dante of “The Sopranos” — usually “takes nice naps during musicals,” says his wife, Maureen. But not at “Rocky” last week.

“Rock ’n’ roll and boxing, we got a lot in common,” Jersey-raised Van Zandt tells The Post. “It’s the way you get out of the neighborhood.”

Any doubts that the show was drawing a different kind of crowd — 68 percent of theatergoers last year were female, reports the Broadway League — were flushed away by intermission.

“Love that I work on a Broadway show where the line to the men’s bathroom is longer than the women’s,” tweeted Whitney Britt, a marketing manager. During at least one preview, the men’s line was more than twice as long as the women’s.

Angel Toscano, 28, came to town from Austin, Texas, last week. He’d never seen a musical before, but when he saw “Rocky” tickets, the former amateur boxer bought one. He says he’s not ashamed to tell friends back home that he now likes musicals.

“Especially ‘Rocky’!” he says. “ ‘Rocky’ is one of the ones that’s OK.”