Yes, he is indeed left-handed. Go to Baidu to search Justin Bieber, and some just show that he is left-handed.

Bieber took a pen in his left hand and signed it for his fans.

The following are all left-handed.

Actor (singer)

Lady gaga, American pop singer Justin Bieber, global pop singer Eminem, American pop rapper Bill Gates and so on.

He is one of them, but I heard that lefties are smart and Justin is smart, too.