Treatment of osmocene with mercuric acetate (10 equiv) in refluxing 1,2-dichloroethane for 18 h affords decakis(acetoxymercurio)osmocene in 84% yield. This is the first description of the mercuration of osmocene. Reaction of decakis(acetoxymercurio)osmocene with halogenating agents affords decachloroosmocene (43%), decabromoosmocene (48%), or decaiodoosmocene (61%). These complexes constitute the first decahaloosmocenes. The X-ray crystal structure of decachloroosmocene is described.