"This came out of the blue for me," Long is quoted as saying by the Daily Star. "But once I'd come and had a look at the training facilities and met the manager, it seemed like the right decision.
"It was a hard decision for me to make, but when you come down to Southampton you see how big a club it is and how well set up it is.
"You look at how well they did last season and they are developing a new squad for this year to try and achieve those accomplishments again, and hopefully I can be a big part of that."
Long will have the chance to make his first Southampton appearance this Sunday when his new team visit Liverpool.