The 29-year-old, who spent the first half of the season on loan at St Mary's from Italian side Cesena, has put pen to paper on a two-and-a-half-year deal.
Guly, who has scored eight goals so far this term, told the club's official website: "The football and the fans make me happy here and I'm very happy with the new contract."
Boss Nigel Adkins added: "He's grown stronger and become part of the team so we are delighted we've been able to secure him on a long-term deal."
Source: PA
Source: PA