We report the observation of cluster glasslike properties in a double perovskite ruthenate Ba₂CoRuO₆ through structural (neutron and synchrotron x-ray diffraction), magnetic, and transport measurements. The system exhibits classic glassy characteristics such as a frequency dependence in ac susceptibility, aging and memory effects, along with the persistence of short-range correlations up to room temperature. The significant (∼30%) antisite disorder on the dimer sites, coupled with the inherent geometrical frustration, allows a variety of exchange (both antiferro- and ferromagnetic) interactions to be distributed randomly across the lattice. On cooling, locally dominant interactions cause spins to nucleate and form local, short-range ordered clusters which grow in size until a global freezing occurs at about T_f ∼ 43K.