Chris Brown At Rihanna's Birthday Party?!

Rihanna's true birthday is February 20th, but she held a party early to celebrate with people like Bruno Mars, Chris Martin from Coldplay, Katy Perry, and more. But one guest in particular made the others sign agreements saying he "wasn't at the party." That guest is Chris Brown!

Sources tell TMZ that Chris refused to enter the private party Monday night at the famous Hearst Mansion in Beverly Hills until everyone signed an NDA -- sending his henchmen in beforehand to gather every partygoer's signature.

Once they were all signed -- most of them at least -- we're told Chris walked in ... and spent a huge chunk of the night nuzzling up against Rihanna.

According to sources, the two got real intimate -- sitting next to each other, even holding hands.

Chris and Rihanna sitting in a tree...again? We'll see. What do you think?