Southampton finally seal deal for Ramirez
Ramirez was linked with a move to the club earlier in the transfer window, with a work permit in place and personal terms signed, only for the deal to fall through.
The player, however, agreed a four-year contract just hours before the window closed.
"A lot of hard work has gone in... with a lot of negotiations behind the scenes to get this deal done," Southampton manager Nigel Adkins told the club's website.
"Great credit to everyone because we have a young, exciting talent joining us who is one of the best around. We have got one of the most exciting prospects in world football coming to St Mary's."
The 21-year-old Ramirez scored 15 times in 60 appearances for Bologna, having joined the Serie A side from Penarol in 2010.
Ramirez has been capped by his country 10 times and is in the squad for next month's World Cup qualifiers against Colombia and Ecuador.