Investigators said Friday that they believe eight people were involved in the attack and are looking for the other four.
The 15-year-old girls were allegedly drugged and raped by several men at a party in the seaside village of Marconia di Pisiski in the southern region of Basilicata.
The four arrested were aged between 19-23. He has been charged with rape and aggravated injury.
The alleged rape took place in the early hours of Tuesday in the villa’s garden in which the party was being held.
Two young women suffered cuts and injuries during the sexual assault.
Pietro Argentino of the prosecution said, “The girls tried to protect themselves and one of them caught the attacker in one of the intimate parts.”
One of the girls is believed to be a British citizen of Italian descent, who has relatives in the area.
The other girl, also British, had a friend who was on vacation with her.
After the alleged rape, the distraught girls stated what had happened to the elder sister of one of them.
“They went back to the party, found the sister, locked themselves in the bathroom and told her everything,” Mr. Argento said.
The teenagers were taken to Madonna Delle Grazi Hospital in the nearby town of Matera, where doctors called the police. He was treated for his injuries and shock.
“What happened to him is of an extreme gravity,” said Giuseppe Rago, a lawyer who works for the teen.
“Girls are strong and strong and I hope they can give evidence with strength and clarity.
“Initially they were unable to remember all the details, they were in shock and still under the influence of hallucinations that they had inadvertently consumed.
“We hope that the authorities will be able to bring justice to all those who have been put through the girls and their families.”
The mayor of the town appealed to the witnesses to come forward for the alleged attack.
Viviana Vary said the community suffered a setback from “hateful violence” against the two minors.
“I feel obliged to make an appeal to the whole community: let’s stand together.”
She said “anyone capable of helping investigators eliminate the case” should contact the police.