In this study, the low temperature synthesis of dense titanium diboride (TiB2) compacts by reaction spark plasma sintering (RSPS) of ball milled mixtures of titanium and boron powders is presented. The influence of milling time and sintering temperature on the in-situ reactive synthesis, densification, grain size and mechanical properties of TiB2 was investigated. TiB2 compacts with relative density above 99% have been obtained at a significantly low sintering temperature of 800 °C. X-ray diffraction analysis confirmed the formation of TiB2 along with a small amount of secondary TiB phase. The effect of TiB on the grain size and the hardness and fracture toughness was investigated. TiB2 compacts processed were found to have a nanohardness > 26 GPa and an elastic modulus > 570 GPa and indentation fracture toughness more than 3.30 MP a.m1/2. It is shown that RSPS can be an attractive route for synthesis of ultrahigh temperature ceramics at low temperatures. © 2014 Elsevier Ltd.