The 26-year-old has been a key player for Neil Warnock's side since signing in July 2007 but has made the move down to League One amid concerns over Palace's finances. The central defender has penned a contract that will keep him at St Mary's until 2013.
Fonte said: "I was happy at Crystal Palace and I only have good memories of my time there but I have made a decision that I am very happy with.
"This might seem like a step back but it may be two steps forward and that's how I see it.
"I want to be in the Premier League as soon as I can and this could be the fastest way to get there.
"Southampton has the stability which Crystal Palace does not have at the moment and I made this decision because the situation at Palace is delicate and I have to think about my future."
Source: Team Talk
Source: Team Talk