The new alleged victim, referred to as Minor Six, was included as part of an updated federal indictment.
Prosecutors also said they had seized more than 100 electronic devices, such as smartphones, iPads and hard drives, after executing a search warrant.
The judge set a trial date for 13 October, six months later than planned.
The Chicago case includes several counts accusing Kelly of child pornography. The chart-topping US singer, whose full name is Robert Kelly, has consistently denied the allegations.