The 26-year-old has agreed a four-year deal with the club and joins for an undisclosed fee from Scottish champions Glasgow Celtic.
Forster recently travelled with the England squad to the 2014 FIFA World Cup finals, where he worked with Southampton’s new Senior Goalkeeping Coach, Dave Watson.
The Northumberland-born custodian began his career with Newcastle United, and had loan spells at Stockport County, Bristol Rovers and Norwich City before joining Celtic in 2010.
After winning three league titles and two Scottish cups as well as gaining UEFA Champions League experience during his time in Glasgow, Forster now moves to St Mary’s ahead of the 2014/15 campaign.
First Team Manager Ronald Koeman said: “Fraser is a very talented player and we are delighted to be able to bring him to Southampton. This is another important part of our rebuilding process ahead of the start of the season.
“The fact that he has worked with Dave Watson before means we already know all about his strengths and weaknesses, and I am confident that we will be able to help him improve even further.
“Fraser is the perfect fit to help complement the players we already have, and he will add a lot of quality to the squad.
“He is an exciting English player with a very good reputation within the game and, at 26, he will enjoy the best years of his career at Southampton.”