Lallana, who has made 34 appearances for England, returned to Southampton last season, having made his name there during his first spell at the club.
He made just five starts last season for the Saints, who were relegated to the Championship, but he also worked as a coach, a role he will continue to fill at St Mary's Stadium under new manager Will Steele.
Lallana joined Liverpool from Southampton in 2014 for €31 million and won the Premier League and Champions League with the Reds.
Lallana came through Southampton's youth academy before making his first-team debut at the age of 18, in 2006.
During two separate spells at the club, he played almost 300 games for Southampton and was part of the team that achieved two consecutive promotions from League One to the Championship, and shortly afterwards to the Premier League.
He was the team captain when he moved to Liverpool, during the summer in which he also played for England at the 2014 World Cup.
After winning the Champions League with Liverpool in 2019 and the Premier League title the following season, Lallana moved to Brighton, before returning to St Mary's in 2024.
"I'm proud of my playing career and I have no regrets," said Lallana.
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