The England international made 265 appearances for Southampton after making his debut in 2006.
It is understood that Lallana, who will wear the number 20 jersey next season, has moved to Anfield for a fee of £25m.
"I'm so excited for the next chapter of my career, coming to a special club like Liverpool. I'm quite humbled and I just can't wait to get started," Lallana told Liverpool's official website.
"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."
The length of Lallana's contract has not been disclosed.