The 26-year-old has penned a contract until the summer of 2021.
He said, “I'm very happy. It's a great feeling to be a part of this club.
“Lots of things attracted me. I know Southampton from watching the Premier League from when I was small, I've always regarded it as the most important league in the world.
“I obviously know the coaching staff, I've worked with them in Argentina and I also spoke with some players who were former teammates in Argentina, who are now playing in the Premier League and they told me all about the club and gave it a shining reference. Therefore, I had no doubts when I took the decision."