The 23-year-old, who played in Portugal's 1-0 friendly win over Italy in Geneva on Tuesday, cost a fee of 6.5 million euros ($7.41 million).
"I joined this club because I believe we can do better and better every year," Soares told the club website (www.saintsfc.co.uk).
"Southampton is a big, big club in England and is very respected. It's a club with a lot of history and, for me, it's very important to play for a big club."
The Saints, who finished seventh in the league last season, also signed Spanish striker Juanmi from Malaga on Tuesday.
(Reporting by Michael Hann, editing by Tony Jimenez)
Share
