The 24-year-old has moved to the Saint Mary’s Stadium to fill the void left by Jannik Vestergaard’s departure to Leicester City.
Lyanco is Southampton’s eighth summer signing, joining the likes of Adam Armstrong, Theo Walcott, Romain Perraud, Thierry Small, Armando Broja, Dynel Simeu and Valentino Livramento.
Lyanco had been with Torino since the summer of 2017, having joined from Sao Paulo and made 23 appearances in Serie A last season. He also spent a season-long loan at Bologna in the 2018/2019 season.
"I'm very happy to be here. It's a dream that I have had since I started playing football as a child to play in the Premier League and for a team like Southampton,” Lyanco said after completing his move.
"It's a dream not only for me but for all my family too.
"I grew up in Brazil and the football is different there, then when I went to Italy I learned that you have to be focused more on tactics.
"My dream was to play in the Premier League and play strong football, and for me English football is perfect."
Lyanco has played youth football for both Brazil and Serbia, where his grandfather was born.
