Fernandez, 29, passed a medical on Thursday and has signed a three-and-a-half year contract.
Fernandez joined Celta in 2012 after helping Velez Sarsfield win the Argentine Clausura title 2011. He played 121 games, scoring 10 goals in Spain.
"He's a player of proven experience in the league who will bring quality and competition to a great midfield such as ours. His incorporation will help us face the demands of having to play three competitions," Atletico Sporting Director Joss Luis Perez Caminero told the club's website (http://en.clubatleticodemadrid.com)
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Atletico are currently second in the Spanish league behind Barcelona on goal difference, face PSV Eindhoven in the last 16 of the Champions League and Rayo Vallecano in same stage of the Spanish Cup.
(Editing by Justin Palmer)
