Southampton manager Russell Martin said: "Firstly, he's a brilliant character. He's got a real edge to him. He's got a fantastic personality, a lot of energy and was a really important member of our group at Swansea, not just on the pitch but off it as well.
“In terms of on the pitch, he's an outstanding footballer who has a brilliant understanding of what we want. He played in a few positions for us previously, and he's coming here as a left-back because he ended up performing as one of the best left-backs in the division last year.
“We've beaten competition from two clubs in the Premier League for him, which I'm really pleased about, because I thought the lure of that would make it difficult for us, but he's come here because he knows the work and he believes in it.”