The 23-year-old joins Sounders from Danish side FC Midtjylland.
“Christian is a player that we have been following for quite some time,” said Sounders FC technical director Chris Henderson.
“He is a good one versus one dribbler who can provide consistent quality service to our strikers. He has good speed and physical presence.”
“This is an exciting opportunity for me, to be joining such a club as Sounders FC,” said Sivebaek.
“I am looking forward to playing in MLS and for one of the best and biggest teams in America. I am very impressed by the professionalism of the club, the high quality of players and staff, and the great number and passion of their fans.”