Mechanical properties and in vitro biocompatibility of graphene/hydroxyapatite (HA) composite synthesized using spark plasma sintering (SPS) are reported in this study. Raman spectroscopy corroborated that graphene nanosheets (GNSs) survived the harsh processing conditions of the selected SPS processing parameters. A 1.0 wt.% GNS/HA composite exhibits ∼80% improvement in fracture toughness as compared to pure HA. GNS pull-out, grain bridging by GNS, crack bridging and crack deflection are the major toughening mechanisms that resist crack propagation. In vitro osteoblast growth tests illustrate that the added GNSs contribute to the improvement of both osteoblast adhesion and apatite mineralization. Therefore, the GNS/HA composite is expected to be a promising material for load-bearing orthopedic implants.