The 27-year-old was wearing all black with an L.A. Dodgers baseball cap on his way out of the gym in Los Angeles.
Rob's trainer Gunnar Peterson told Us Weekly, "He's getting it done. He works hard, and I love having him in here. He's no stranger to the grind."
Kardashian has kept a low profile since late May, when he was spotted flying home from France ahead of Kim’s Florence nuptials to Kanye West. Earlier this month, mom Kris Jenner spoke about her son on The Today Show.
"Rob is working on himself," she said of her only boy. "He's going to be okay. A lot of prayer and, you know, he's such a great human being … he's my heart."
For the past few months Rob has been struggling with his weight, acknowledging his drastic fluctuation on Twitter after he landed in Europe for his sister’s wedding.
"To the blogs saying I went to fat camp or rehab, LOL,” he tweeted. "Then why am I still fat u fools?? Y'all must have run out of real news. First, I don't believe in rehab or therapy or anything of that sort and second, I'm a grown man and will work on myself when I choose. I’m aware that I'm fat that def ain't a surprise to me lol… My only therapy will be in the gym, anyways had to say somethingggggggggg."
Rob has since deleted all of his tweets, starting fresh on social media after Kim and Kanye said their “I Do’s.” He is also currently overseeing the construction of sister Khloe Kardashian’s house.
On June 10, Khloe tweeted, "Having my brother oversee my construction at our new house while I'm in NY is a tad scary. I'm way too controlling lol."
Rob later replied, “I got u don't you worry. Got the boom boom room all set up and everythangggggg."