Odell Beckham Jr. will NEVER leave the NY Giants -- so says a world-famous porn star ... who's laying her booty on the line for OBJ!

Richelle Ryan -- star of XXX hits like "Big Wet Butts" and "Plump Rumps" -- is a HUGE Giants fan ... so when we got her leaving Vivid's HQ in L.A., we asked her what to make of the G-Men reportedly trying to deal Odell.

Richelle was triggered -- and told us there ain't a chance she'd let that happen.

"He's not leaving the Giants ... I'll make sure of that," Ryan told TMZ Sports. "I will work my magic."

Richelle also gave us the low-down on her viral "gangbang" pic with UNLV's baseball team ... including whether or not any of the young bucks joined her in the Mile High Club!!