Fans have long awaited Sammi's return and we've got a first look at how it went down when she showed up, without warning for the OG cast and others, on "Jersey Shore: Family Vacation."

The teaser for the 2nd half of season 6 starts with Ronnie Ortiz-Magro -- Sammi's ex-bf -- talking to The Situation about coming back, then it cuts to Sammi's strolling into the house ... leaving Pauly D, JWoww, Snooki and Vinny in complete shock!

Some of 'em were speechless with their jaws on the floor ... while Pauly D said, "Is this real? Are you a hologram? Is this AI?"

As we reported, Sammi's comeback was a well-kept secret ... producers kept the cast in the dark in order to capture this awesome moment fans are finally getting to see.

As you know, It's been over a decade since she's worked on anything "Jersey Shore"-related, ... she was the only OG cast member not to return to 'Family Vacation'.

Most folks suspected she didn't sign on for 'Family Vacation' because of her past with Ronnie ... whom she dated on and off throughout the original series. He ended up stepping away from the show indefinitely 2 years ago.

And clearly, there's a lot in store -- hence Ronnie talking about his potential return. Remember, we got pics of Ronnie talking to producers while filming at the Margaritaville Resort in Orlando, and we know Sammi was there too.