Dejan Lovren has completed his £20m transfer from Saints to Liverpool.
The defender passed a medical this weekend and has signed a "long-term" deal at Anfield.
Croatian international Lovren leaves a year after Saints paid £8.5m to sign him from French side Lyon.
The deal takes the total amount received by the club in transfer fees this summer to about £70m - with that figure set to rise.
Liverpool had already had two bids for the centre half turned down, before a fee was agreed late last week.
Lovren made his own intentions clear following the end of the season, when he handed in a transfer request.
The 25-year-old also considered not returning for pre-season in a bid to force through a move, while he conducted an interview in his home country in which he said he felt as if his head was already at Anfield.
Lovren is the third player to make the move from Saints to Liverpool this summer, following the departure of Rickie Lambert and Adam Lallana.
He is also the latest in a string of high-profile stars to leave St Mary's, with Luke Shaw having already joined Manchester United, while Calum Chambers is set to leave for Arsenal in a £16m deal.
After completing the move, Lovren told Liverpool's official website: "It's a dream come true. I'm happy and glad to be here. I'm proud and can't wait to start. I know we have millions and millions of supporters - Liverpool is more than a club.
"After I played at Anfield last season, I said to myself: 'One day I hope I'll be able to play here for Liverpool'. The fans will know from the first minute that I'm a player that will give 100 per cent on the pitch."