The former Barcelona man has signed a two-year contract with the newly-promoted side and becomes their fifth summer signing.
Valdes departed United after making just two
appearances for the club under Louis van Gaal, having had a public
falling-out with the Dutchman.
Valdes was accused of refusing to
play for the club’s Under-21 side by Van Gaal, a claim he strenuously
denied. He eventually made three appearances for the youth squad before
spending the second half of last season on loan at Standard Liege.
The Spaniard won the World Cup with his country in 2010,
and was also a member of La Roja’s European Championship-winning squad
in 2012, while he lifted six La Liga titles at Barca, along with three
Champions League trophies.
Middlesbrough have made significant efforts to reshape their squad this
summer, acquiring Viktor Fischer from Ajax, Bernardo Espinosa from
Sporting Gijon, Jordan McGhee from Hearts and Marten de Roon from
Atalanta.