Coming up on the main stage ... Heeeeeeeidiiiiiiii Montag!!
TMZ has learned ... The former MTV reality star has struck a $25,000 deal to appear at the Crazy Horse III strip club in Vegas on October 19 ... but there's a pretty big catch.
Montag -- and her G-cup boobs -- will not be getting naked. So, patrons will only get a chance to throw money at giant unfamous boobs that night.
Montag was only hired to host the club's 3rd Year Anniversary Party.
Sources at the club tell us ... Spencer will accompany Heidi to the gentlemen's establishment.
Fun Fact -- the club will be serving sushi that night.