High sintering temperature (∼2000 °C) and high assistant pressure (≥30 MPa) are usually required for the densification of high-entropy diborides. Herein, dense (Hf0.2Mo0.2Ta0.2Nb0.2Ti0.2)B2 ceramics were successfully prepared with metallic Ni as additives by pressureless sintering at 1800 °C. Results showed that the relative density of (Hf0.2Mo0.2Ta0.2Nb0.2Ti0.2)B2 significantly increased from 66.93% to 96.17% as 10 wt% Ni was introduced. It was found that the liquid phase with good wettability to transition metal borides was formed during sintering, leading to the acceleration of densification. With the increase of Ni content, the grain size of (Hf0.2Mo0.2Ta0.2Nb0.2Ti0.2)B2 increased (from 0.74 μm to 12.30 μm) while the hardness increased first to a maximum and then decreased. Among all samples, (Hf0.2Mo0.2Ta0.2Nb0.2Ti0.2)B2 ceramics with 7.5 wt% Ni additives exhibited the highest hardness of 20.44 ± 0.99 GPa.