The transfer fee agreed by Southampton for the 26-year-old midfielder is believed to be around £25million (31million euros) and he will be on a five-year contract.
Lallana is Brendan Rodgers' third signing of the summer following to Anfield his Southampton 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."