Valencia confirmed on Wednesday that they had agreed a deal with the Portuguese side to take Otamendi to Mestalla for a reported fee of €12 million plus add-ons.
The Argentina international has signed a five-year contract with Juan Antonio Pizzi's men, and will link up with compatriots Pablo Piatti and Federico Cartabia.
Otamendi, 25, was quick to express his delight at the switch, which will see him in the footsteps of legendary Argentina striker Mario Kempes, as well as fellow countrymen Roberto Ayala and Pablo Aimar, by gracing the Mestalla.
He told the club's official website: "After playing for a club like Porto, very prestigious and large, it is an honour for me to come now to a club like Valencia, one of the most historic and attractions in Spain and Europe.
"I am very happy for this step in my career.
"I know that we Argentines have been an important part of the history of Valencia.
"Wearing the shirt that was once defended by Mario Kempes, (Roberto) Ayala and (Pablo) Aimar and many idols is a responsibility that I take with great joy and enthusiasm."
Otamendi will officially join Valencia on July 1.
The centre-back signed for Porto in 2010 from Velez Sarsfield and has won the UEFA Europa League and three Portuguese crowns in his time at Estadio do Dragao.
