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.
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."