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A plastic deformation process was demonstrated to self-join hydroxyapatite (HA), fabricating pore-free joints at 1275°C at a strain rate of 10−5 s−1. To determine optimum joining conditions, high-temperature compressive deformation of HA was investigated for strain rates between 5 × 10−6 and 10−4 s−1 at temperatures 1175–1275°C. X-ray diffraction revealed primarily the HA phase with the presence of tri- and tetra-calcium phosphate phases. Steady-state flow stresses were 0.6–45 MPa and increased with increasing strain rates. Stress exponents of ≈1 indicated a viscous diffusion-controlled deformation mechanism with an activation energy of ≈354±36 kJ/mol. Absence of cavitation and grain shape changes was consistent with grain boundary sliding.