Two accounts tweeted claims about incidents alleged to have happened in 2014 and 2015.
The account of one anonymous accuser has been removed from Twitter, but Justin Bieber responded to it, saying: "There is no truth to this story."
The second account is still online. This account is not anonymous.
A spokesperson for Justin Bieber told Newsbeat they wouldn't comment and had nothing to add to the singer's tweets.
Over the weekend, Riverdale actors also denied accusations of a sexual offence made by other anonymous accounts.