The 26-year-old underwent a medical at the Reds' Melwood training ground last Friday after Southampton accepted a bid, believed to be £25million, for the player.
Liverpool spent the weekend finalising the details of Lallana's contract, understood to be five years in length, and have now confirmed the deal has been concluded after all the necessary paperwork was completed.
Lallana is Brendan Rodgers' third signing of the summer, following to Anfield Saints team-mate Rickie Lambert and Bayer Leverkusen midfielder Emre Can, whose move from Germany will be ratified on Tuesday with the official opening of the transfer window for registrations.
"I'm so excited for the next chapter of my career, coming to a special club like Liverpool," Lallana told liverpoolfc.com.
"I'm quite humbled and I just can't wait to get started.
"Hopefully I can settle in quickly, and I will give it my best throughout the whole time I'm here.
"I hope I can add something to the team and hopefully we can win trophies while I'm here, because that's why I'm here - for success."
OUR VIEW: It's hard to assess Lallana's true value. Three years ago he was a League One player, but he is fresh from a season in which he made the Premier League team of the year and earned a World Cup call-up. All told, £25 million seems a decent price for an Englishman who has just turned 26 and is just entering his prime years.
Lallana's intelligence and creativity make him a good fit for Brendan Rodgers' Liverpool side, who need to bolster their squad depth ahead of their return to the Champions League. What's more, the player seems genuinely delighted at the move, and will be desperate to prove himself given he is a relatively late bloomer.
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- Brendan Rodgers
- Rickie Lambert