The England international is undergoing a medical on Merseyside today.
The 26-year-old only arrived back in England earlier this week and the speed in which these latest events have occured confirm that his participation with the national team at the World Cup was holding up the deal.
Precise details of the contract length and wages are yet to be disclosed but manager Brendan Rodgers will at least be satisfied that he has secured his number one target for the summer.
Lallana is seen as a perfect fit for the Northern Irishman's style of play, where pressing from the front is high on the agenda.
Another part of his appeal is the fact that he is already accustomed to the Premier League and will not need time settling in or adapting in comparison to a foreign import.
Whether that means LIverpool end their pursuit of mooted targets Alexis Sanchez and in particular, Xherdan Shaqiri, time will tell.
Sanchez could be included in any deal that sees Luis Suarez head in the opposite direction to the Nou Camp.
At the least Lallana is versatile and tricky, a commited player keen to play at the highest level who can offer Philippe Coutinho some much needed competition for the number 10 role.
Liverpool need two players for every position if they are to adequately challenge on both domestic and european fronts in the forthcoming campaign after all.
Lallana's arrival is a sign of intent from the Premier League runners-up, who look like they are serious of going one better in 2014-15.
How new Southampton incumbent Ronald Koeman replaces the midfielder should be interesting but with £25million to play with they won't miss out on any targets for want of money.
The club have already lost Rickie Lambert to Liverpool while Luke Shaw has gone to Manchester United so they will need to use the money they have gained from their transfers wisely otherwise a season of struggle could await.
Koeman has some big shoes to fill in Mauricio Pochettino who, of all the departures to happen this summer, will probably be the keenest felt at St Mary's.