A seething boyfriend landed 25 years behind bars Tuesday for fatally stabbing his girlfriend’s ex in a twisted love-triangle slaying at a swanky Long Island yacht club, prosecutors said.

Victor Panebianco, 32, of North Carolina pleaded guilty to first-degree manslaughter charges last month in the Nov. 10 killing of 52-year-old Christopher Millwater at the ritzy Stony Brook Yacht Club, where Millwater was a member, the DA’s office said.

The killer and his gal pal, Kacy Corso — who previously briefly dated Millwater before things went south — drove from North Carolina to ambush the victim, authorities said.

“The killing of Christopher Millwater at the hands of the defendant was brutal,” Suffolk County District Attorney Ray Tierney said in a statement.

“We are grateful that the court imposed the maximum sentence requested by my office. We hope that the victim’s oved ones find some solace in knowing that justice was serviced.”

Prosecutors said Millwater had arrived at the yacht club, where he liked to fish, around 3:40 a.m. on the day he was killed. Panebiano and Corso were already laying in wait for him in the parking lot, authorities said.

Millwater tried to make a run for it, driving around the killers’ car to make for the exit, but Panebianco crashed into him to trap him, with the victim then attempting to flee on foot.

Surveillance footage showed Panebianco giving chase “while swinging a knife” and eventually catching up to Millwater and plunging the knife into his chest twice, prosecutors said.

Millwater’s chilling last words were, “You’re stabbing me?“

Police responded to a 911 call at the club and found Millwater’s body, while Panbianco and Corso drove back to their apartment in North Carolina, where they were later picked up by the Stokes County sheriff.

Corso, 33, a former DoorDash and Uber driver, had a fling with Millwater but grew increasingly hostile after they split, at one point calling him a “grave robber” in an unhinged social-media post.

Millwater meanwhile took to Facebook himself to write several macabre posts just days before his death.

“Plenty of time to rest when I’m dead so now it’s a quick shower and gym time,” he wrote on his page.

The next day, he was online again, this time hinting at a troubled past — including the three-sided recovery-unity-service symbol connected to recovery and sobriety.

“NOBODY can throw my past in my face,” the post said. “I will personally sit down and tell you my story, beginning to end. What may be your gossip, is your testimony. I’ve made peace with everything.”

Suffolk County prosecutors said Corso pleaded guilty to first-degree manslaughter July 22 and is scheduled to be sentenced to seven years in prison when she faces the judge Wednesday.