The 25-year-old Argentina international passed his medical on Monday and has signed a four-year contract at the Liberty Stadium.
The transfer is subject to international clearance but, once completed, Fernandez will fill the void left by Chico Flores, who departed for Qatari outfit Lekhwiya earlier this month.
Fernandez could go straight into the Swansea squad for Saturday's Premier League clash with Burnley.
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