Graphene oxide and zirconia powders were mixed using a colloidal coating route. In situ reduced graphene oxide-toughened zirconia ceramics were prepared by spark plasma sintering. Their microstructure, mechanical properties, and toughening mechanisms were investigated. The results show that graphene oxide can be easily reduced in situ during sintering and that it disperses homogeneously within the zirconia substrate. Compared with the toughness of 3 mol.% yttria-stabilized zirconia, the fracture toughness of in situ reduced graphene oxide-toughened zirconia increased by up to 175% (from similar to 6.07 to similar to 10.64 MPam(1/2)) at 0.09 wt.% graphene oxide with a small increase in hardness. The improvement is more significant than that of prereduced graphene oxide-toughened cases, and it is associated with the formation of a C-O-Zr bond at the interface in addition to conventional toughening mechanisms.