Rap-A-Lot Records J. Prince -- Rewards Las Vegas Stranger ... Thanks for Finding My $75k Watch!

Hip-hop honcho J. Prince nearly came up snake eyes in Las Vegas -- losing a $75,000 diamond encrusted watch in the airport -- until a stranger came forward to save the day ... and then Prince made his.

Sources connected to the Rap-A-Lot Records chief -- and infamous Drake defender -- tell us he had just gone through TSA Tuesday night when he noticed his iced out Hublot was missing.

With $75k on the line ... you better believe Prince's crew immediately scoured the terminal - and while they were searching, we're told a dude walked up and whipped out the watch. Witnesses say Prince's security surrounded him to confirm it was the right one, and that's when the awesomeness happened.

In the video ... you see Prince walk up to the good Samaritan and hand him something. We're told he reached into his pocket for some loose change to reward the guy -- and by loose change we mean ... $500!!

So grateful. So baller. Jewelry recovery is really becoming J. Prince's thing -- right, Stevie Francis?