The former Barcelona shotstopper arrives at the Riverside as a free agent after his contract at Manchester United came to an end last month.
Boro head coach Aitor Karanka has expressed his delight at having the experienced 34-year-old on board ahead of their return to the top flight.
You can't imagine how delighted I am because it's not usual to bring players like him to recently promoted teams," Karanka told the club's website.
"I'm really pleased because he is really committed. All 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."
Prior to joining United in 2015, Valdes enjoyed heaps of success at Barca, winning six La Liga titles and three Champions League crowns in 12 seasons as a first-team regular at Camp Nou.