Mitchell Jones, 23, of Nyton Road, Aldingbourne near Chichester, appeared at Portsmouth Crown Court yesterday.
He admitted three counts of sexual activity with a child aged 14 and one count of sexual activity with a child aged 15.
The charges related to his offending against a 14-year old girl in Chichester over three months in early 2015, and also with a girl aged 15 in Hampshire around the same time.
Jones was remanded in custody and will be sentenced on April 28 at the same court.
He has been told he will receive a custodial prison sentence and he will also be put on the sex offenders register.
Detective Constable David Midgley said; “Jones contacted the girls separately via social media and, despite knowing their ages, exploited them for his sexual gratification.”
Anyone with concerns on learning this and similar cases can contact Sussex Police via firstname.lastname@example.org or call 101.