The 27-year-old forward, who scored 16 goals in 29 Serie A appearances last season, has signed a four-year contract with Mauricio Pochettino's side
The 27-year-old striker, who worked under Mauricio Pochettino At Espanyol, has signed a four-year deal with the Premier League club.
Osvaldo, an Argentine-born forward who has been capped nine times by Italy, scored 16 goals in 29 Serie A appearances last season.
Osvaldo is Southampton's third signing of the summer transfer window following the arrivals of Dejan Lovren from Lyon and Victor Wanyama from Celtic.
Pochettino's latest acquisiton could make his debut next Saturday in Southampton's Premier League fixture against Sunderland at St Mary's.