The 32-year-old, currently with the England team in Miami preparing for the World Cup, scored 13 league goals for the Saints last term as the south coast side ended their campaign with a club-record 56 points.
"I can't believe it. I've loved this club all my life. I left here 17 years ago - and I haven't stopped loving it since," Lambert said on the club website.
"I have always dreamt of playing for Liverpool, but I did kind of think the chance of playing for them had gone. I didn't think the chance would come," he said.
No transfer fee was stated but British media had reported Lambert, a boyhood Liverpool fan who spent his youth career at the club, was expected to cost around four million pounds ($6.71 million).


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