Corie Adams, formerly of Matlby, Rotherham, pleaded guilty at Sheffield Crown Court to 11 sexual offences against two girls aged 13 and two aged 14.
The 18-year-old had earlier admitted an additional six offences against three girls aged between 12 and 14.
Adams is due to be sentenced on 2 December.
The offences took place between January 2015 and early 2016.
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Adams was arrested and charged following a joint operation by South Yorkshire Police and Rotherham Council.
Det Insp John Fitzgibbons said Adams' offending was "prolific and incredibly shocking".
"We were able to identify a number of his victims but there may well be more young girls out there yet to come forward," he said
"While he has admitted his part in the sexual abuse of many young girls in Rotherham, this does not lessen the awful impact his hideous crimes have had on their lives."
Det Insp Fitzgibbons said Adams had preyed on the girls' vulnerability and age for his own satisfaction.
"This is a truly despicable crime," he said.
Adams admitted six counts of sexual activity with a child, one count of rape, three counts of causing a child to engage in sexual activity, four counts of inciting a child to engage in sexual activity, two counts of causing a child to watch a sexual act and one count of grooming.
A man has admitted a string of "hideous" sexual offences against seven teenage girls.