The Chile international has been added to the Viola's squad, with the Serie A side believed to have paid a fee of around €4 million (£3.1m) for the midfielder
The transfer has been on the cards for the last few days, with the attacking midfielder arriving in Florence on Friday to undergo a medical.
Fernandez, 26, had only one year remaining on his deal with Sporting, and the Portuguese giants have opted to sell the playmaker.
The fee for the deal is undisclosed, but reports in Italy speculate that the Viola have invested around €4 million (£3.1m) in the ex-Villarreal star.
It is expected that Fernandez will be unveiled to the fans and media in the coming days.