Zirconium diboride (ZrB2) is an important ultra-high-temperature ceramic, which exhibits outstanding mechanical properties and is widely used in extreme environments. Extensive experimental studies, however, have found that synthesized ZrB2 specimens show widely scattered indentation hardness values ranging from 8.7 to 26 GPa. We have performed comprehensive stress-strain calculations of ZrB2 to explore its structural and stress responses and found that ZrB2 possesses an intrinsic indentation strength of 32.7 GPa, which is on par with those of other transition-metal borides that exhibit higher indentation hardness values of ∼30 GPa. This result suggests that large variations in measured hardness are driven by extrinsic factors, and an analysis of available experimental data indicates that the quality of the crystallinity of specimens holds the key to realizing improved hardness corresponding to the predicted intrinsic indentation strength. These findings offer insights into the origin of the previously reported lower hardness values of ZrB2 and raise the prospects of achieving superior strengths in well-crystallized ZrB2 that approach or match those of other ultrahard transition-metal compounds.