The 28-year-old has signed a long-term contract with the Gunners and joins the club for an undisclosed fee, believed to be around £6million.
Arsenal boss Arsene Wenger told the club’s official website: “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.”
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.”
