Gareth Vincent Hall, 22, from Talysarn near Caernarfon, was arrested in Chicago in May 2015.
On Tuesday in Oregon, he admitted four counts of rape, two of serious sexual offences and one of online sexual corruption of a child.
He is due to be sentenced on 1 March. A kidnapping charge was dropped.
Local television station KMTR in Oregon said Hall could face between a minimum of 25 years in prison and up to 50 years behind bars.
The FBI had appealed for information last year after he was arrested, because it was concerned he had travelled to other US states.
According to the FBI, Hall had flown to Oregon last April where he hired a car and then took the girl to a hotel.
A separate investigation is under way in Wales, where Mr Hall worked for Gwynedd council at a leisure centre in Caernarfon.
After his arrest in the US, it emerged North Wales Police had arrested a man in October "on suspicion of offences committed online" and was bailed.
He was suspended from his post at the Arfon Leisure Centre, where he also taught children to swim, pending the outcome of the inquiry.
He is no longer employed by the council, which has said it followed "all relevant child protection procedures".
The council said "all employees who work with children and young people must comply fully with Disclosure and Barring Service (DBS) checks", adding it had received no complaints about Hall from users of the leisure centre.
A lifeguard from Gwynedd faces up to 50 years in prison in the United States after admitting raping a 10-year-old girl.