The youngster has signed a deal that will see him spend the next few years at St. Mary’s Stadium.
The Brazilian left-back signed a pre-contract agreement with the Premier League side in the summer.
But the move was finalized this winter, when the Brazilian league season for Sao Paolo ended.
“I'm very happy to be here, representing this club. I'm really happy to be here playing in the Premier League,” he said.
“I always try to give my best when I step onto the pitch, so the team can count on me for anything.
“I’ll do whatever I can to help, both on and off the pitch. The fans can trust that I’ll always give my all with determination and heart. I’ll fight for this club for as long as I’m here.
“I’ll do everything I can to honour this shirt. In the short time I’ve been here, just being here with my teammates and wearing this shirt has been a unique experience for me.
“The fans can expect the best from me, and I’m confident we’ll be happy here.”