In the snap, Rob is surrounded by Kim, Kendall Jenner, Khloe and Kourtney Kardashian. All of the sisters are lavishly dressed for the occasion, while Rob wears jeans, a black sweatshirt, baseball cap and sneakers.
Kim wrote in her caption that she was "40 and feeling so humbled and blessed."
"For my birthday this year, I couldn’t think of a better way to spend it than with some of the people who have helped shaped me into the woman I am today," she added. "Before COVID, I don’t think any of us truly appreciated what a simple luxury it was to be able to travel and be together with family and friends in a safe environment."
For Kim's birthday on Oct 21, Rob did share a throwback photo of himself and his big sis on his family-run Instagram.
Rob, who hadn't been part of Keeping Up With the Kardashians in years, also made a rare on-camera appearance on the latest episode. The entire Kardashian family turned out to celebrate Kim and the brother was there to watch old home videos of the birthday girl.
As they looked back at some of her most iconic fashions and watched a sweet montage of her relationship with husband Kanye West, Rob quipped, "Remember when they called you guys 'Kimye?'"
Earlier this summer, Khloe said during an appearance on SiriusXM’s The Morning Mash Up that Rob was "coming back around" to potentially appearing on the family's reality show again, opening the door for the possibility of some appearances on KUWTK's upcoming 20th and final season.
"He's feeling more confident and comfortable. I think he just started a whole new season, so here we go," she said at the time.
While speaking with ET in September, Caitlyn Jenner also teased Rob's return to the front of the cameras.
"Rob was an integral part for the show. He's kind of stayed out of the limelight for the last few years, which is fine, it's his own prerogative," she said. "I hope during this -- well, they're still shooting, I think until November, the final season -- [that we'll see him]."
"I have seen him a lot, and the times I've seen him, he's been doing much better," she continued. "I'm happy for him."
That same month, a source told ET that he was in a really great place and dating again.
"Rob is really happy and doing great. He’s dating again," the source shared, adding that the pandemic had helped Rob put things into perspective and get back control of his life.
"His number one priority has always been and continues to be his daughter, [Dream]. Family is everything to Rob,” said the source, who added that she is the biggest motivation for him to get healthy.