The mechanical properties of a nominally phase pure ZrB2 ceramic were measured up to 2300°C in an argon atmosphere. ZrB2 was hot pressed at 2000°C utilizing borothermally synthesized powder from high purity ZrO2 and B raw materials. The relative density of the ceramics was about 95% with an average ZrB2 grain size of 8.8 µm. The room temperature flexural strength was 447 MPa, with strength decreasing to 196 MPa at 1800°C, and then increasing to 360 MPa at 2300°C. The strength up to 1800°C was likely controlled by a combination of effects: surface damage from oxidation of the specimens, stress relaxation, and decreases in the elastic modulus. The strength above 1800°C was controlled by flaws in the range consistent with sizes of the maximum ZrB2 grain size and the largest pores. Fracture toughness was 2.3 MPa·m1/2 at room temperature, increasing to 3.1 MPa·m1/2 at 2200°C. The use of higher purity starting materials improved the mechanical behavior in the ultra-high temperature regime compared to previous studies.