Koster, a defender, became the first Dutch player to sign with a WPS team and adds depth in the middle of the defense, where 2008 player-coach Christie Rampone, who is pregnant, will be absent for the start of the 2010 season.
Koster first played for the Dutch national team at the age of 16 and has since been capped 91 times. She has spent the past two years with AZ in Alkmaar, winning another pair of league championships.
"It has always been a dream to play in the USA, and I am very delighted that this dream comes true at Sky Blue FC," Koster said of the signing. "I think WPS is the best competition in the world at this moment, and I look forward to the challenge of playing with and against the best players in a professional setting."
Koster played for the Oranje at the 2009 Women's Euro, where it was the surprise semifinalist.