The 29-year-old began his career with the Seagulls, making 155 first-team appearances before leaving to join Colchester United in 2008.
"He knows the club, he has experience, he has presence and he knows the division. He's a winner and he has quality," Brighton manager Gus Poyet told the club's official website.
"We have a lot of quality in our midfield. We need that presence in midfield. The fact it was Brighton helped him make a decision. It's important that the fans know how much he wanted to come back.
"He is a real man. We wanted that extra experience and presence on the pitch and in the dressing room. He shows what we were looking for. We didn't know he was available and as soon as we knew there was a small chance we went for it and waited for it to happen."
Hammond's current Southampton contract expires in June 2015.