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.
Porto signed Meireles in 2004 from Boavista and he made 137 appearances for the club as well as becoming a regular for the national side.
He is new Liverpool manager Roy Hodgson's fourth major signing since taking over from Rafael Benitez.
Meireles joins Joe Cole, Christian Poulsen and Milan Jovanovic at Anfield.
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