The newly-promoted side have signed the former Barcelona and Manchester United goalkeeper on a two-year deal.
Middlesbrough manager Aitor Karanka has told the club's official website that he is delighted to have been able to sign Victor Valdes, stressing that the goalkeeper is a real coup for the promoted side.
Valdes has signed a two-year deal at the Riverside to become the newly-promoted team's fifth major signing of the summer as they prepare for a return to the Premier League.
Though he will turn 35 midway through this season, the Spaniard will undoubtedly bring a touch of class to the Boro side, and boasts a wealth of experience in winning teams.
Valdes played 535 times for the Barcelona first team during a 12-year spell with the club, occupying the number one spot for the vast majority of that period.
After winning 21 trophies with the La Liga giants, including six league titles and three Champions League triumphs, he left to join Manchester United two years ago but made just two appearances for the club as David De Gea continued as the undisputed first choice.
After being released by United he has chosen to remain in the Premier League by signing a two-year deal with the Teesiders, and Karanka admitted his pride at securing the signature of one of the most decorated goalkeepers in football.
The Boro boss told the club's website: "You can't imagine how delighted I am because it's not usual to bring players like him to recently promoted teams.
"I'm really pleased because he is really committed. All the the conversations we've had he knows why he is here and he is coming to add to what we have.
"He knows how strong this group is. It's a privilege for us to be able to bring a player like this to the club."
Valdes joins fellow arrivals Bernardo Espinosa, Viktor Fischer, and Marten de Roon at the Riverside as Karanka looks to assemble a team capable of surviving in the top flight this season.
Valdes will likely take over from another veteran stopper, 37-year-old Dimitrios Konstantopoulos, in Karanka's starting XI.