"He is very happy to join Southampton and obviously believes he can contribute to them this season," said agent Barry McIntosh.
"It is a tremendous move for him and he is delighted to have concluded the deal.
"He has always had hopes of coming to England and he is very happy. He hopes to use his speed and skills to show what he can do and pave the way for some of his friends to join him.
"He would love the chance for his best friend Luciano Figueroa to join him."
Arias, a left sided player, is said to have signed a three-year-deal for an undisclosed fee from Velez Sarsfield.