Shaun Sutherland, 16, of Perth, told police he had sex with the girl on two occasions on the same evening, although she had no memory of the incidents.
At the High Court in Edinburgh the teenager admitted two charges of rape, by having sex with a child who has not reached the age of 13, last year.
Advocate depute Jane Farquharson said: "The accused was part of a group of young people, most of whom are 16 years or younger, who met up regularly at weekends in an area known as the South Inch."
The prosecutor said drinking was a feature of their gatherings with cider, vodka and Buckfast tonic wine being consumed.
Ms Farquharson said the girl had drunk before but on that evening consumed vodka for the first time: "She describes feeling tipsy, light headed and a bit dizzy and that she found it difficult to judge her actions. The girl has no recollection of what happened."
Sutherland admitted to having sex with the girl.
The girl's mother was later contacted about entries that had been spotted on social media site Facebook regarding the youngster and Sutherland.
When interviewed by police Sutherland admitted having intercourse with the girl.
The judge, Lord Bannatyne, deferred sentence on Sutherland until next month for the preparation of a background report.