The 24-year-old was USL-1 MVP in 2007 after leading the Sounders with 22 points. He also scored five U.S. Open Cup goals.
He had previously played for Lorient in the French second division.
"Sebastien is an outstanding young soccer player with loads of upside," said General Manager Adrian Hanauer. "He has tremendous physical capacity, he is very good technically and tactically, and has the potential to be a very effective MLS player. Sebastien is also a great individual, and has the kind of character that our fans will be proud of."