Christopher Melvyn Colebrook, aged 44, of Bowers Fold, Doncaster town centre, pleaded guilty to two counts of rape and one count of sexual assault against a 16-year-old girl in Doncaster last year.
He also admitted two counts of rape and one sexual assault against a 19-year-old man in Doncaster last June.
Colebrook also pleaded guilty to assaulting a police officer.
A South Yorkshire Police spokeswoman said: “Colebrook targeted vulnerable teenagers, plying them with alcohol before subjecting them to vile and horrific sexual offending.
“The victims have been incredibly brave in sharing their ordeals with police.
“We take this crime incredibly seriously and will do whatever we can to bring these despicable individuals before the courts.”
He is to be sentenced next month.