Upon reaching the milestone, the 17-year-old singer sent out a series of celebratory tweets.
"I love u… changing my life everyday," he wrote. "I'm from a town of 30,000… so u guys are 333 times my town! CRAZY! NeverSayNever."
And so on.
Lady Gaga accumulated 10m fans over two weeks ago, being the most followed user on the site. After Bieber, the top five is rounded off with Barack Obama, Britney Spears and reality 'celebrity' Kim Kardashian.