The 27-year-old made 37 La Liga appearances last season, but was unable to help Osasuna avoid relegation.
And he has now opted to join a whole host of new arrivals at Porto, who finished a disappointing third in the Primeira Liga last term.
"I am very glad to be in a club as great as Porto," said the Spaniard.
"It is a club with a huge history, battling for European competitions and want to achieve titles. It is the biggest challenge of my career.
"I consider myself a keeper very hardworking and do all I can for my teammates and the team."
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