Boro beat off stiff competition to the land the Barcelona legend after an 18-month spell with Manchester United came to an end earlier this summer.
The 34-year-old brings a wealth of experience, having made over 500 appearances for Barcelona during a trophy-laden career that has included a World Cup winners medal in 2010, a Euro 2012 winners medal, three Champions League successes, and six La Liga titles.
The capture of Valdes is seen as a real coup for the club, and Head Coach Aitor Karanka said: "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, who has signed a two-year contract with the club, becomes the fifth signing of the summer, following Viktor Fischer, Bernardo Espinosa, Marten de Roon, and young central defender Jordan McGhee into the club.