Rumours have started circulating the web after several videos appeared on YouTube in recent months linking the 16-year-old with the secret society and suggesting that his music videos are sending out Illuminati messages.
Conspiracy theorists believe the Illuminati are the real people in power, secretly ruling the world and influencing events through government, institutions and big corporations.
Bieber refuted the rumour on Twitter, insisting he was a Christian.
"I have not joined the Illuminati or any other cult. I'm a Christian and I pray before every show and am thankful for every blessing,” he wrote.
He also dismissed rumours that his mum had been offered $50,000 to pose topless in Playboy magazine.
"My mom is a moral woman ... let's just leave that one for what it is... because that rumour just grossed and weirded me out."
He added that he was very much alive.
He tweeted: "I'm not dead. I had to check this one... but it turns out I'm alive."