After Southampton revealed the Italy international had put pen to paper on a four-year deal, his former employers in the Italian capital released the details on a deal initially worth £12.9m. That figure could rise by another £1.7m dependent on Osvaldo's performances at St Mary's.
The fee for the 27-year-old, who is due to be presented to the media tomorrow, exceeds the £12.5million they forked out last month to bring midfielder Victor Wanyama from Celtic.
Osvaldo worked with Southampton manager Mauricio Pochettino when the pair were at Espanyol, although getting him to the south coast took some persuasion.
"Southampton football club is delighted to announce the signing of Italy striker Pablo Daniel Osvaldo," a statement on the club's official website, www.saintsfc.co.uk, said.
Osvaldo, who has nine Italy caps, becomes Southampton's third signing of the summer, following an £8.5m move for Lyon defender Dejan Lovren and the signing of Kenya midfielder Wanyama.
Southampton won their opening match of the season at West Brom on Saturday, with a late Rickie Lambert penalty securing a 1-0 victory.