The Good American founder turned 40 on Thursday, and she shared a small snippet of her birthday celebration on social media ... where Rob made an unexpected cameo.

In a video uploaded to Instagram, Khloe is seen enjoying an intimate birthday dinner with family and friends -- including brother Rob, BFF Malika Haqq, and grandmother Mary Jo "MJ" Campbell.

Rob appears to be having a grand ol' time at the dinner ... cheering on his big sis as she's presented with a beautifully decorated birthday cake.

This is the second time this month the ultra-private Rob has made a surprise appearance in the Kardashian-Jenners' very public life.

Remember, the Arthur George Socks founder appeared on a recent episode of "The Kardashians" ... though, it was over the phone. So, we're not really sure that counts as a real cameo.

Nonetheless, Rob willingly had a NSFW conversation about his penis with Khloe ... who was trying to acquire sperm for bestie Malika.

While these appearances don't make for a full return to the spotlight, it proves Rob isn't hiding away ... like he was known to do years ago.

Still, Khloe and Rob have always had a close bond, being the youngest Kardashian sibling and all. Rob seems to be signaling something with his actions lately ... he'll do anything for his big sis!!!