He simply captioned the pic, "Gnight."
It is unclear if the photo is a throwback or new. If it is a new snap, it shows that Rob's training sessions with his sister Kim are helping him lose weight. He has been avoiding the spotlight for years since gaining over 100 lbs.
Rob and Khloé's trainer Gunnar Peterson recently said that Rob is working hard to get fit. He told People, "Rob is always in! He's slammed schedule-wise and still makes this work. Never worry about Rob."