The England international has joined Liverpool on a long-term deal, believed to be worth around £25m, after passing a medical at Melwood.
Lallana made 250 appearances during his time at Southampton, becoming club captain in 2012, and hopes to play a key role as the Reds pursue silverware next season.
"I'm so excited for the next chapter of my career, coming to a special club like Liverpool,” he told the club’s official website. “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."
The 26-year-old becomes Liverpool’s second signing of the summer, joining Rickie Lambert who also arrived from Southampton, while a deal in principle has been agreed for Bayer Leverkusen's Emre Can.
Manager Brendan Rodgers is confident that Lallana has all the attributes needed to become a success at Anfield.
"We are delighted to have secured the services of one of the Premier League's top talents in signing Adam Lallana," said Rodgers.
"What is apparent about Adam is that his character is on a par with his talent.
"He is an exceptional young man and someone who has a love for the game - and, most importantly, a desire to progress and improve.
"Adam is motivated by being the best he can be and being part of something special. We believe he fits right in with what we are trying to do at Liverpool.
"We have seen, over the past two seasons in the Barclays Premier League, he has a tactical awareness to adapt to what is required of him and to put the team above his personal ambitions.
"He has leadership skills and personal qualities that make him a special commodity and I am delighted we have acted decisively to make sure he is wearing a Liverpool shirt next season.
"Adam is aware there will be great competition for places at this club and he is hungry to improve further. We are confident he will fit perfectly with the culture we have at Liverpool.
"I'm excited about working with Adam and helping him to progress even further."