4:53 PM PT -- Rob just uploaded an image of himself on social media for the first time in over a year, and he's showing off his slimmed-down figure in his Halloween costume alongside Kris Jenner.
It's the only picture Rob has of himself on his entire Instagram page, so this is very rare.
Rob Kardashian's stepping back out into the public eye ... for another one of his sister's birthday parties.
Rob was spotted entering Delilah in WeHo well after 1 AM to celebrate Kendall Jenner’s 24th bday, decked out in an orange hoodie and his signature blue baseball cap. As he walked in, he put his hands around the waist of Kylie's current BFF, Stassie Karanikolaou.
We're told Rob was at Blind Dragon for a while beforehand ... along with a massive celeb guest list partying with Kendall on Halloween.
As we told you ... Rob was seen for the first time in a long while last week on Kim's Instagram Story while he was celebrating with the family at her 39th birthday dinner. He's looking a lot slimmer.
Our Kardashian sources told us he'd lost weight by eating healthier, exercising, and most significantly ... quitting alcohol.
It's unclear if Rob abstained from drinking as he was hanging with his sis at the Hollywood hot spots, but at least he's joining the party.