The Shame singer has been working with trainer Nicky Holender to get in shape.
According to British newspaper The Sun, Holender said: "He's gone from being overweight and not looking so good to being an athlete, he's worked his a*** off."
Nicky has even got the 36-year-old to cut down on chocolate.
He said: "He has become very careful about what he eats and in four months he now has a full six-pack going on.
"All I'm doing is getting him to eat well and then he can have his sweet stuff on his cheat days, he has so much muscle."
And now Nicky, who also helped former Spice Girl Mel B get in shape, is turning his attention to Robbie's smoking habits.
He said: "It's the best thing he could do for his body, he's come such a long way now and done so well that quitting the fags would be the biggest goal."