Boro used Twitter to announce the signing after a long day of waiting for the club's expectant fans, posting a video of Valdés' name being printed on the back of their new goalkeeper kit.
The immensely experienced Spaniard will be expecting to take the number one shirt at the Riverside Stadium, but will have to dislodge current first-choice Dimi Konstantopoulos ahead of the new Premier League season.
Valdés brings list of honours to Teesside
Valdés joins with a wealth of experience at the highest level of the game, holding Barcelona's record for the number of appearances made by a keeper.
His trophy cabinet is a formidable one, boasting six La Liga titles, two Copa del Reys, six Spanish Super Cups, three Champions Leagues, two UEFA Super Cups and two Club World Cups at club level alone.
He has also lifted the World Cup and European Championship with Spain, and won the Ricardo Zamora Trophy for the best goals conceded-to-games ratio in the Spanish top flight on five occasions.
Boro's transfer spree continues apace
Valdés has been confirmed as Middlesbrough's fifth signing of the summer, following the arrivals of Marten de Roon, Viktor Fischer, Bernardo Espinosa and Jordan McGhee.
The club, preparing for their first season back in the top flight, are also waiting on the decision over Gastón Ramírez' work permit, with club chiefs confident of a positive outcome.
Aitor Karanka will now switch his focus to strengthening his side's attacking options. It has been reported that the Spaniard is stepping up his search for a new forward, but reports linking the club with Robin van Persie are thought to be incorrect.