Long joins Saints from Hull City, where he scored four goals in 15 matches last season. The 27-year-old has also played for English clubs Reading and West Bromwich Albion, as well as Cork City in Ireland.
"Southampton play a brand of football that’s enjoyable to watch, and I’m sure it’ll be enjoyable to play in as well," Long said in a statement. "I think the formation they play will suit me, and the manager has a lot of knowledge about football and I’m looking to learn off him."
Southampton opens the Premier League season on Sunday with a match at Liverpool.
- Dan Gartland