Zac Efron’s name just got pulled into an explosive federal sex trafficking trial against the wealthy Florida-based Alexander brothers … with prosecutors claiming they used ties to famous people to lure victims.

In a Manhattan courtroom Tuesday, the government’s first witness — testifying under the pseudonym Katie Moore — told jurors a dream invite to a 2012 party at Zac’s NYC apartment spiraled into a nightmare hours later … when Alon Alexander — the brother of of real estate moguls Tal and Oren — allegedly raped her repeatedly at their home, and mocked her afterward.

Katie testified she was offered alcohol at Efron’s place and later took what she believed was Molly with Tal and her friend … from there, she says they hit a nightclub, where she was given another drink — and said she remembered little until waking up naked in a different apartment with Alon, also naked, standing over her.

She told the jury she tried to get up but was pushed back down, as she said, “I don’t want to have sex with you.” According to her testimony, Alon laughed and replied, “You already did,” before allegedly assaulting her again.

Katie said she barely interacted with the “High School Musical” star, and is not accusing Zac of any wrongdoing.

Katie is one of dozens of women accusing Tal, Oren, and Alon Alexander of rape, sexual assault, and trafficking. Assistant U.S. Attorney Madison Smyser said in opening statements the brothers “masqueraded as party boys when really they were predators.”