Arsenal have signed Brazilian international left-back Andre Santos from Fenerbahce for £6.2m.
The 28-year-old has signed a long-term contract at Emirates Stadium and will wear the number 11 shirt.
Manager Arsene Wenger said: "Santos is a quality player, with the ability to both defend well and get forward."
Santos, who played at the 2011 Copa America, said: "It was one of my dreams to play for a major European football club and I have accomplished this."
After Sunday's 8-2 defeat by Manchester United, Wenger said he was chasing new players before the transfer window closed on 31 August.
Wenger added: "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."
Last season Santos made 25 league appearances for Fenerbahce, scoring five goals helping them to the Turkish championship.
He said: "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."
Fenerbahce announced the transfer to the Istanbul Stock Exchange, quoting 7m euros [£6.2m] for the deal.
Santos played for Brazil as they won the Fifa Confederations Cup in 2009, but he did not make the 23-man squad for the 2010 World Cup Finals. He did return to play in the 2011 Copa America, but missed a penalty as Brazil lost to Paraguay on spot-kicks in the quarter-finals.
He is back in Istanbul to say goodbye to team-mates, coaches and the fans.
Fenerbahce were withdrawn from the Champions League by the Turkish Football Federation following match-fixing allegations and have sold many of their high-profile players.
Wenger has been looking for a left-back after Gael Clichy joined Manchester City earlier this summer, while on Tuesday he allowed .
He played 21-year-old Kieran Gibbs in that position on the opening day of the season against Newcastle United. but he then injured his hamstring in the Champions League qualifying play-off first leg against Udinese, leaving Bacary Sagna and then Traore to fill in.
Meanwhile, 20-year-old Arsenal forward Gilles Sunu has signed a four-year deal with French first division side Lorient.
The French under-20 international said: "My main aim was to join Lorient so that I can play.
"If I get to play up front I hope to score 10 goals this season."
Arsenal have also sent Costa Rica striker Joel Campbell on loan to French club Lorient until the end of the season.
The 19-year-old striker has only just joined the Gunners from Deportivo Saprissa but the north London club wants him to get first-team experience in France.