The 21 year-old attacker is a highly-rated prospect who has been on the wishlist of several leading European outfits, with Saints seeing off top clubs in both Italy and the Barclays Premier League to land the youngster.
The deal to bring Ramírez to St Mary's was quickly agreed and processed over ten days ago with a work permit appeal being successfully granted last Friday.
Only the finer details of the transfer prevented his signing from being completed and, with those matters now taken care of, Ramírez has signed a four-year contract at St Mary's with Saints paying an undisclosed fee to Serie A side Bologna.
First Team Manager Nigel Adkins said: "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."
Ramírez has already been selected in the Uruguay squad for their next two 2014 FIFA World Cup qualifiers, which are due to take place during the upcoming international break.
Uruguay travel to face Colombia on Friday 7th September and then host Ecuador on Tuesday 11th September, after which the forward will return to Southampton ahead of the trip to Arsenal the following Saturday.
In Profile: Gastón Ramírez
South American-born Ramírez moved into European football just two years ago when he signed for Bologna from Peñarol in his homeland.
In that short time he became a key player for the Serie A outfit as they finished 9th and 16th in Italy’s top flight during his time there.
Comfortable both on the wing and as a central attacking option, six-foot-tall Gastón is an exciting player who likes to drive forward with purpose and has the ability to create opportunities for his teammates.
Ramírez has already been capped ten times by his native Uruguay at senior level having made his debut aged 19 during a 7-1 win over Indonesia in October 2010.
He was also involved at this summer’s London 2012 Olympic Football Tournament, playing against Team GB and new Saints teammate Jack Cork.
Ramírez impressed throughout that game at the Millennium Stadium, going close to hitting a last-minute equaliser with a rising shot from 25 yards out which clattered the crossbar.
Prior to that match, the Uruguayan had already caught the eye with a stunning free-kick that levelled the scores against the United Arab Emirates in the opening game of the tournament at Old Trafford.
Last season Ramírez made 33 league appearances for his club, scoring eight times and making four assists as Bologna secured a top-half finish in Serie A.
Overall, he played for the Rossoblù on 60 occasions, finding the back of the net 15 times.