"Santos is a quality player, with the ability to both defend well and get forward and help with attacks. He has experience in the Champions League and for Brazil and has proven he can deliver at the highest level. We welcome Santos to the Club and look forward to him having a big impact for us," the Frenchman told the club's official website.
Santos added: "I am delighted to have signed for Arsenal. It was one of my dreams to play for a major European football club and I have accomplished this. I am very happy to be part of this team. All the Brazilians who came to this club spoke wonders about it and I am delighted to be part of the Arsenal family."