The 24-year-old spent last season on loan at Sunderland, making 13 appearances in all competitions, but has now signed a four-year contract for an undisclosed fee.
"Sebastian was an important part of the team under Dick (Advocaat) last season and we are delighted that he has joined us on a permanent basis," sporting director Lee Congerton told the club's official website (www.safc.com).
"He has Barclays Premier League experience, which is important and, after spending last season on loan with us, he will be able to settle into the team very quickly."
Sunderland narrowly avoided relegation last term -- finishing 16th, three points above the drop zone.
(Reporting by Tom Hayward; editing by Ken Ferris)
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