The Uruguayan international playmaker arrives at the Saints for a club record fee from Serie A, adding to the considerable attacking reinforcements brought to the South Coast
The left footed playmaker has played both on the wing and in a central attacking position, and arrives on a four-year contract from the Italian side for a club record fee thought to be in the region of £12 million.
Manager Nigel Adkins told the club's official website: "A lot of hard work has gone in from the Football Club with a lot of negotiations behind the scenes to get this deal done.
"Great credit to everyone because we have a young, exciting talent joining us who is one of the best around.
"The Chairman has gone and backed this deal big-time with the financial outlay, and what we have got is one of the most exciting prospects in world football coming to St Mary’s.
"There is a lot of ambition at the Football Club and we are all delighted that Gastón is going to come and join us."
Ramirez scored ten goals in 35 appearances for the Rossoblu last season, and earlier this summer featured at the London Olympics with Uruguay, for who he has won 11 international caps.
The Saints have added considerable attacking firepower over the summer to complement Rickie Lambert, signing forward Jay Rodriguez for a then record of £7 million from Burnley, and this week adding Zambia international striker Emmanuel Mayuka from Swiss side Young Boys.