FAITH HILL knew Taylor Swift was “trouble” as soon as she walked in!

Dressed in a skintight little black dress, the “22” singer slinked across the stage in front of Faith’s hubby Tim McGraw at a recent concert – and her seductive antics didn’t go unnoticed by Tim OR Faith. After catching Tim ogling the much-younger star, Faith, 45, cornered Taylor and warned the 23-year-old “tartlet” to stay away from her man.

“THERE WAS MAJOR SEXUAL chemistry onstage between Taylor and Tim,” said a source. “And Faith was fuming backstage.”

On April 8, Tim hosted a star-stud­ded multigenre music event at the MGM Grand in Las Vegas. Among the talent he assembled for his “Su­perstar Summer Night” show, which will air on May 19 on CBS, were Nelly, Pitbull, Ne-Yo, Jason Aldean, Lady Antebellum, Keith Urban and Taylor, who ditched her usual con­servative attire for a super-sexy black mini-dress with see-through cutouts.

“Taylor chose that dress specifically so that she could parade in front of Tim,” said the source. “Tim couldn’t take his eyes off of her, and Taylor seemed to be enjoying the attention.

“She couldn’t take her eyes off him, either.”

Despite her youth, Taylor has earned a reputation as a serial dater, racking up an impressive list of ce­lebrity boyfriends, including John Mayer, Joe Jonas, Jake Gyllenhaal, Taylor Lautner and RFK Jr.’s hunky son Conor Kennedy.

Meanwhile, Taylor collaborated with Tim, 45, on his new single “Highway Don’t Care.”

And while he’s older than her usual crushes, he’s in better shape than ever thanks to a grueling workout regimen.

“Knowing Taylor’s reputation for bouncing from man to man, Faith made sure to put her foot down right away,” added the source. “She made it clear to Taylor that Tim is OFF-limits.”