The England midfielder has long been linked with a move to Anfield and the deal has now been confirmed by the Merseyside club, with Lallana signing a long-term contract at Anfield.
Lallana, who featured for England in the World Cup in Brazil, spoke of his delight at joining such a "special" club.
"I've been granted my wish to play at such a fantastic football club, in Champions League football and competing for Premier League titles," the 26-year-old said upon joining the Merseyside club. "I'm so excited for the next chapter of my career, coming to a special club like Liverpool. Last season, deep down, I was thinking: hopefully one day I'll be able to play in that team. That dream is reality now."
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"We're delighted to have secured one of the Premier League's top talents," said Liverpool manager Brendan Rodgers. "I'm excited about working with Adam and helping him progress even further."