Hardness was measured for six different nominally pure and single-phase high-entropy diboride ceramics with the AlB2 structure that were produced by a two-step process. Most of the ceramics had relative densities greater than 99.5%. Vickers hardness average values ranged from 21.0 GPa to 25.4 GPa at a load of 9.81 N, while values increased to the range of 33.1 GPa to 48.3 GPa at a load of 0.49 N. Among the compositions, the highest hardness values at a load of 0.49 N were 48.3 ± 2.3 GPa for (Cr0.2Hf0.2Ta0.2Ti0.2Zr0.2)B2 and 45.8 ± 5.3 GPa for (Hf0.2Ta0.2Ti0.2W0.2Zr0.2)B2, both of which are high enough to be considered superhard. The present study demonstrated that high-entropy diboride ceramics are a new class of superhard materials.