The 21 year-old arrives for a £15m fee and has penned a five-year contract.
Fernandes said: “I’m very happy to be here, it’s a dream come true to come to England and play in the Premier League. It’s the best league in the world, in my opinion. It has the best coaches, the best players, the best teams. It’s good for me, for my growth and I’m happy.
“I know some things about Southampton. I know about the city. I know José Fonte played here, Cédric, so I want to talk to them. I spoke to the manager and I like him a lot – his energy, his feeling, he’s very good. He talked to me and I’m very happy.
“I want to help the team, but I’m new in England so I need help from them too! I want to help the team and enjoy the Premier League with the guys this season. I think I can help when we have the ball and when we don’t have the ball.”
Saints manager Russell Martin also said: “Mateus is a player we have been monitoring for a while and someone we are really excited about. He brings great energy, retains possession well and always looks for the forward pass.
“He is young and full of enthusiasm to come and play for Southampton. With the ability he has to drive forward from midfield, we think he has some key attributes to really help us – not just in the future, given his age, but here and now in the Premier League.”