The Premier League club said on their website they had paid 14 million euros for the 27-year-old.
Meireles, who started all his country's matches at the World Cup in June, is known for his dynamism and will help fill the gap left by the departure of Javier Mascherano to Barcelona.
He is new Liverpool manager Roy Hodgson's fourth major signing since taking over from Rafael Benitez.
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