Sources with direct knowledge tell TMZ ... the "Vanderpump Rules" star is set to become the new host of "Love Island USA" next season -- meaning she'll be replacing Sarah ... who's been in the driver's seat for the past two seasons (Season 4 and 5).

In terms of why this is happening -- our sources tell us Sarah is leaving because she took another job opportunity that she wanted to pursue ... and it just so happened to be shooting during the same time frame she'd be needed for 'LIUSA.'

We're told an announcement with the news is forthcoming ... although, we're told cameras haven't started rolling. Our sources also say the deal isn't officially done just yet -- but barring any major hiccups ... we're told this is expected to be signed, sealed and delivered.

While it remains to be seen if Sarah will return as host in the future, we're told there's no indication at this point that she'll be back.

A bummer for SH fans, no doubt, but this is a dream come true for Ariana -- who's known as a devoted fan of the reality show ... and who even made a cameo on 'Love Island' last season.

Of course, this is also just the latest gig Ariana has booked as she continues her hot streak post-Scandoval. Remember, she got hired for a stint on "Dancing with the Stars," and was also recently doing "Chicago" on Broadway. Now, she's on to a whole new reality series.

Before Sarah came aboard to host 'Love Island USA' ... the host was Arielle Vandenberg, who was the face of the franchise for three whole seasons. As part of a major rebrand, though, in terms of the show's formatting ... they pivoted, and tapped Sarah instead.

'Love Island USA' also went from airing on CBS to Peacock, its current home.