The 21-year-old playmaker has signed a four-year contract at St Mary's with the Barclays Premier League club revealing that a verbal agreement was in place last Friday.
A statement on saints.co.uk read: "Saints are pleased to confirm the signing of Uruguayan international Gaston Ramirez from Italian side Bologna."
Saints manager Nigel Adkins added: "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 Gaston is going to come and join us."
Source: PA
Source: PA