Leonardo DiCaprio & Rihanna -- Gettin' Hot At Playboy Mansion

It wasn't Leonardo DiCaprio's birthday this weekend ... but it sure felt like it ... he was seen going straight up "Wolf of Wall Street" on Rihanna at the Playboy Mansion.

It's a good thing for Leo ... designer Nikki Erwin had boyfriend/producer Chuck Pacheco -- "Alpha Dogs," "My Sister's Keeper" -- invite him to her star-stacked 30th B-Day at the Playboy Mansion Saturday ... and our spies tell TMZ, Cap and RiRi were gettin' super steamy and even swapping spit.

A friend of Leo's tells us the kissing didn't happen ... but 2 people who were there say it did. BTW ... not that there's anything wrong with it.

The two apparently seemed very comfortable getting hot and heavy around stars like Paris Hilton, Hilary Duff, Tom Hardy, Pamela Anderson, Naomi Campbell, and Tobey Maguire.

It's not clear if they left together but we're told they ended up at the same after-party -- along with 50 Cent -- and things got even hotter.

Make a wish.