But not content with conquering the music charts, the singer and former boyfriend of Britney Spears has his heart set on taking Hollywood by storm.
The 25-year-old star has landed the lead role in gritty new movie Alpha Dog, which opens today, one o
f three films he'll be seen in this year.
"I'm really inspired by acting," he enthuses. "I don't know if that sounds sort of New Agey or whatever, but it's true. I've been really lucky to have a musical career that has gone pretty good and acting is something I've always wanted to do."
Justin joins an esteemed cast in Alpha Dog, including Sharon Stone and Bruce Willis, but he admits he wasn't fazed by his A-list co-stars.
"When I came on to the film all the other actors were probably not looking forward to having to deal with me, but I wasn't worried about it and I just showed up to work the same as everybody else.
"If you go in with the right intentions then the outcome ends up OK."
It's hardly surprising though that the star is taking it all in his stride – after all, he's been performing from the age of 13, when he became a Mouseketeer in the US show The Mickey Mouse Club, alongside Christina Aguilera and Britney.
At the age of 15 he became a poster boy for millions of girls when he joined the hugely successful American boy band *NSYNC, but insists all that early fame went right over his head.
"I was always so removed from it all," he says with a shrug. "I always cared so much about what I did that that stuff didn't really register. It can mess with your mind a little bit when you're a teenager, but I learned very quickly that the more I didn't pay attention to it, the more I realised you have to remove yourself from it, otherwise it will drive you completely insane.
"My parents instilled in me the mindset of always being humble, so I'd like to believe that all the teen idol stuff went in one ear and out the other. Now when I'm on the red carpet I'm there to do a job, but it's the coolest job in the world," he adds with a disarming grin.
His level-headed attitude seems to be in sharp contrast with troubled ex Britney, who has just left rehab following her famous head-shaving episode.
Though Justin is said to have offered support to his fragile ex, he won't confirm rumours that they will record a duet together. Neither is the modest star particularly forthcoming about his reputation as one of the sexiest singers on the planet.
And despite dating a string of beauties including Britney, Cameron Diaz and Scarlett Johansson, the modest Tennessee-born star reckons he's still clueless about the opposite sex.
"I haven't really learned anything about women," he says with an embarrassed smile. "I'm a man so I have some kind of learning disability, and women wouldn't have us any other way."
Justin becomes much more open when discussing Alpha Dog, a movie which will show fans a very different side to the affable and charming star.
In the film, based on a true story, Justin stars as Frankie Ballenbacher – a rich LA kid who gets caught up in a botched drug deal, resulting in a kidnapping.
Justin admits he relished the chance to show fans his darker side.
"It's a tough movie to watch," he says, "But this is as close to what happened as we felt like we could make it. I don't think it's repulsive, the point was to reveal the truth.
"I think these kids thought what they were doing was fun and cool and I don't feel any of them thought about what was going to happen. The movie is about bad parenting and loss of perspective because of bad parenting."
With two more movies lined up, Black Snake Moan and Shrek 3, it looks as if Justin might have to start juggling his singing career – he's just embarked on a six-month tour – with his newfound movie success. However, the busy star denies he's a workaholic.
"You have to offset all that activity with doing nothing at all and I'm pretty good at that," Justin says with a laugh. "If I'm not working I always do something that's fun and relaxing."
And despite a career that just seems to be getting better and better, the unassuming star says the things that make him truly happy have nothing to do with work success.
"My priorities are my relationship with my mom, my family, my peace of mind, my friendships and my career – in that order," he says with a satisfied grin.
