In the spliced footage, which was originally shown during a 2012 Keeping Up with the Kardashians special, "Dishing It Out," in which the family sat down to watch never-before-seen clips together, Kardashian can be seen laughing uncontrollably, awkwardly dancing and even jokingly slapping himself in the face.
"This makes me ridiculously happy. @robkardashian," Jenner, 19, captioned the post on Tuesday, complete with a heart emoji.
In the footage, family members Kylie Jenner, Kourtney Kardashian, Khloé Kardashian and Caitlyn Jenner (then known as Bruce) also watch the video.
Rob Kardashian hasn't appeared on the reality show for a while, secluding himself after gaining weight and battling depression, sources have told PEOPLE. His mother Kris Jenner revealed in an April episode of KUWTK that he gained more than 100 lbs. this year.
While the 28-year-old has been recently photographed grabbing food and driving his car, he has maintained an extremely low profile. Older sister Kim Kardashian West told Today's Matt Lauer in April that he may just be "going through a phase where he's not comfortable in his own skin."