@justinbiebr / Instagram 1 / 9
@justinbiebr / Instagram 1 / 9

Even Justin Bieber's haters have to admit -- the 20-year-old "Baby" singer is looking super-toned in his steamy new Calvin Klein ads.

Beliebers pretty much had their best day ever when he was officially announced as the newest underwear model for the iconic brand on Tuesday, with Bieber going shirtless and rocking a pair of white CK undies in the very grown-up new shots.

PHOTOS: Celebs In Their Underwear

But just how did he attain such an enviable physique?

According to the skateboard-loving singer himself, he works out five days a week in order to get the noticeable results.

"I work out with my trainer, Patrick Nilsson, five days a week for about 45 minutes each," he reveals in a new interview with Us Weekly. "We do three days splits: back and triceps one day, chest and biceps on day two, shoulder and legs on day three, and abs every other day."

Surprisingly, he didn't have to go on a diet.

"I'm not a special diet, but I do try to watch what I eat," he explains. "I do eat unhealthy sometimes -- I am a 20-year-old guy!"

He also gave credit to the muscle-building supplement MYO-X on Twitter on Wednesday.

Bieber reveals he was actually sick during the top-secret shoot with supermodel Lara Stone, which actually took place months ago.

"It was actually two-and-a half [days] because I got sick during the second day," he tells the magazine. "It was hard to keep things under wraps. ... I've wanted to be involved with Calvin Klein for a while now."

But although the pop star seems to view this campaign as an opportunity to redeem his less than stellar public image -- captioning one of his CK pics on Instagram with hashtags reading "learn from your faults" and "don't be ashamed redeem" -- it appears he can't help but attract the haters.

Another thing garnering some hate -- that Bieber bulge! Plenty of people have taken to social media accusing him of padding his crotch in the shots.

Bieber appears to be taking all the expected criticism in stride, however, and has even got the support of perhaps the most legendary CK underwear model of all-time -- Mark Wahlberg.

"Just spoke to my boy @mark_wahlberg," he tweeted. "Honored to be a part of the legacy. @CalvinKlein #mycalvins."