The 29-year-old midfielder has arrived from Lazio, who agreed terms with Boro last week.
A number of clubs, including Atletico Madrid and Athletic Bilbao, also wanted the former Valencia star, but Boro boss Steve McClaren has managed to lure him to The Riverside.
"We're absolutely delighted to have signed Gaizka," McClaren told the club's official website.
"We have worked extremely hard and beaten a lot of other clubs to bring to Middlesbrough a truly top quality player.
"His signing shows not only what an attraction we now are for top players but just how ambitious we are too.
"I came across Gaizka many times in European competition when I was with Manchester United and he was at Valencia.
"He was undoubtedly one of the best players in Europe at that time. He was so good that Lazio paid nearly £30 million for him.
"As Boro fans know, we've spent all summer searching for a quality player to replace Geremi on the wide right. I'm confident we've got the ideal player in Gaizka.
"He is extremely skilful and we believe he'll bring much creativity to our play, while also contributing towards our goalscoring.
Mendieta has agreed a loan move for the rest of this season with a guaranteed four year contract at the end of the initial spell - should he decide to continue his stint on Teesside.
Boro have also confirmed that there will no fee involved in the transfer of Mendieta.
The move is a major boost for the Teesside outfit, who have struggled to make an impact in the transfer market this summer, but the luring of a man who cost £29 million just two seasons ago is a major coup for the North East club.
He should now make his debut in Boro's Premiership clash with Arsenal on Sunday.
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