Miley Cyrus -- Secret New Project with HOT Porn Star

Miley Cyrus' secret new music video is so sexy ... she and her DJ friend had to hire a smoking hot PORN STAR to do the job right -- TMZ has learned.

The video was shot a couple weeks ago at Beacher's Madhouse at the Roosevelt Hotel in Hollywood -- for Miley's new song "Decisions" with Israeli DJ Borgore.

Sources close to production tell TMZ, the song's about 20-year-old porn star Jessie Andrews -- who used to date Borgore -- so Miley and Borgore asked Jessie to star in it.

Jessie -- who won this year's AVN Award for Best Actress -- happily obliged, and we're told the entire video went off without a hitch. FYI, the pic was taken the night of filming, and shows the dress Jessie wore in the video.

And since the entire thing was filmed at Beacher's -- we're told you can expect a music video filled with little people, a unicorn, a goat, and a woman with watermelon-smashing boobs.