Casey is leaving the team by mutual consent after scoring 50 goals in 121 games for the Rapids. He also was the MVP in Colorado's 2-1 win over FC Dallas in the 2010 MLS Cup.
The 31-year-old Casey has been hampered by injuries the last two seasons. He scored two goals in 2012.
Said Casey: "While I am sorry to see my time with the Rapids come to an end, I want to thank the supporters here in Colorado for everything they gave to me and the club over the last several years."
The Rapids also parted ways with Ian Joyce, Tyrone Marshall, Joseph Nane, Scott Palguta, Tyson Wahl, Luis Zapata and Edu.
(Copyright 2012. The Associated Press. All Rights Reserved)