Fernandez was unveiled yesterday by Viola officials.
"I am happy to be in Italy. I've played in prestigious leagues, but for me Serie A is the most important challenge of my career," said the 26-year-old.
"I've always wanted to play in Italy. Fiorentina are among the biggest clubs and I'm proud to represent them. When I reached Spain I was only 20 years old, but now I think and hope it's the right time.
"I hope to continue the strong tradition of South Americans in Florence, made of such great names. I love to play the ball and to be in midfield, though I can also play behind the strikers."