The 20-year-old has begun a new workout regime after signing up to play a window cleaner in Broadway musical How to Succeed in Business Without Really Trying, according to The Sun.
It is thought that the new role will require Radcliffe to appear shirtless on stage, inspiring him to get a toned torso before the project begins next year.
A source said: "Daniel wants to completely re-sculpt his body and has taken Ronaldo as his inspiration. He envies his muscle definition and is hoping to get a similar six-pack for himself.
"Daniel is taking his fitness regime really seriously. He's stopped drinking, knocked the fags on the head and is watching every mouthful. He put on a little timber last year and is keen to get down to a more svelte shape. He's already lost nearly a stone in the past month."
Radcliffe's first major theatre project was a revival of Peter Shaffer's Equus. He played the lead role of Alan Strang in both the West End and Broadway in 2007 and 2008.