Clarke, 28, has not been a regular in the Glens line-up this season and Crusaders manager Stephen Baxter has been tracking the player for some time.
The Tyrone man will not be available for Saturday's League Cup final as he is cup-tied.
Clarke moved to Glentoran from Newry City in May 2009 and he made 160 appearances for the Oval club.
Glentoran midfielder Richard Clarke has moved to Crusaders on a two-and-a-half-year deal.