Rihanna Photographed Making Surprise Appearance at Friend's NYC Birthday Party

Rihanna made a surprise cameo appearance -- but not in a movie ... instead, the iconic singer showed up at a friend's private birthday party, and we got the pics to prove it!

Check 'em out ... Riri looked as stunning as ever in her form-fitting silver dress, popping into Pergola restaurant in Manhattan early Monday morning as the soiree was already in full swing.

Our sources say Riri had a couple of bodyguards watching her as she enjoyed a fun-filled time with her 20 or so friends.

Everyone, we're told, was on a guest list -- and if you weren't, well, don't even think about trying to get in.

We're also told RiRi was really cutting loose, dancing, laughing, chatting, and taking photos with her posse. TMZ obtained a video showing RiRi boogeying down with one of her buds.

As for her longtime partner, A$AP Rocky, we're told the rapper was nowhere in sight. Instead, he was probably on daddy duty at home, taking care of the couple's 3 young kiddos — RZA, Riot, and Rocki.